This isn't so much a story idea as a proposed background that could be plugged into some story. Despite the references to Love Hina and Omamori Himari, they do not have to be included in said story.
The origin of the group of people known as magical girls stems from the peculiar thaumic geography of the archipelago known as Japan. Situated on a region where numerous ley-lines intersect with each other in close proximity, the islands are capable of supporting a much higher density of magical creatures than any other area in the world. As humans came to settle the islands, they came into conflict with many of these creatures, some of them because they considered the human settlers to be a food source, and some simply because there was a finite amount of land and resources available and they didn't feel like sharing.
Conflicts between the humans and so-called monsters were generally resolved by massive loss of life among the humans until the 9th Century, when a priestess whose name has been lost to history (But is known poetically as Sakura-hime for the large number of cherry trees that grew in the region in which she lived) discovered how to modify a variety of purification rituals to be able to harm the monsters more effectively than crushing them through sheer weight of numbers. Aided by a local samurai, she was able to rid the entire valley in which they lived of monsters. Sakura-hime was the first example of what is now known as a magical girl.
After securing the valley from monster attacks, Sakura-hime married her companion and trained her daughters and various other girls in the art of fighting monsters. These girls then went out to protect other areas from monster attacks. Once the rulers of these other lands realized that the talent for these monster-hunting techniques could be passed down to later generations, they started rewarding the monster hunters by offering them marriages to their trusted retainers and younger sons, thus ensuring that their lands would produce more monster-hunters to continue protecting the area.
This potential for upward mobility inspired a great many girls to seek out a career in monster hunting. Like the countless young men in western tales that went out on adventure in the hopes of winning a knighthood through some act of valor and thus being able to court a lady or princess, these girls went out hunting monsters in the hopes of winning acclaim and rising from being just another peasant to being the wife of a samurai or lesser daimyo. Those monster-hunters who were uninterested in marriage were often retained by the local lord as a samurai in their own right. Not all of these girls followed the practices invented by Sakura-hime - some tried to invent their own. A great many of these died when their experiments proved ineffective, but those who succeeded started new lines of monster hunters.
While many of the lords hoped that these matches would produce magic-wielding samurai, this was not to be. The time and effort required to learn swordsmanship and magic was such that one could master one or the other, but not both. As the men were needed to fill the daimyos' armies, this resulted the sons being trained to be samurai by their fathers, while the daughters were trained to be monster hunters by their mothers. There were some exceptions - men learning magic and women learning the blade (One daughter of the Aoyama clan who insisted on trying to master both ended up developing a way to perform the rituals used by her family through swordsmanship and thus created the art of Shinmei Ryu), but the general rule was that magic was the province of women, which in turn lead to the coining of the term 'magical girl'.
As the feudal era progressed and more and more areas became relatively secure against monsters, the daimyos thought to use the magical girls to further their own power. At first it was simply a matter of offering the services of their magical girls to their neighbors who didn't have any in exchange for various concessions, but then they came up with the idea of using them as weapons of war. The use of magical girls to supplement the military might of their overlords continued until the 16th century, when every single magical girl aligned with the Azai and Asakura clans were killed by the forces of Nobunaga Oda at the battle of Anegawa. This left the Omi province almost totally defenseless against monster attacks, which resulted in the region being devastated in the years to follow (History would blame the damage on Nobunaga's armies as he continued his campaigns in the Omi province, cementing his reputation as a butcher, despite his being only indirectly responsible).
After Nobunaga's death in 1582, those magical girls who had survived the war came to the conclusion that they had failed in their duty to protect the people against monsters because they were too busy fighting each other. They then forged a pact that they would only fight against monsters, not other men, so that their powers would not be so misused again in the future.
The Meiji Restoration of the 19th Century brought the end of the samurai, and with it the livelihood of most magical girls, who by long tradition had been considered part of the samurai class. Unwilling to form into an army under the Emperor like many of the samurai did for fear of being used in ways that would violate the pact that had been sworn in the Sengoku era, they managed to negotiate a deal with the Meiji government in which they would be paid for the monsters they hunted.
This caused a split in the magical girls. Some turned their traditional calling into a part-time career while they sought other jobs that brought a more regular income to put food on the table when there were no monsters to be hunted. The others went out of their way to destroy any monster they could find in order to bring the bounties in. The scoured the islands looking for monsters, driving many of them near extinction. Some of the monsters treated in this way, such as the Mizuchi, had not posed a threat to humans for centuries, but they were exterminated anyway. By the time of the Great Depression, the active magical girls were running out of monsters to hunt, as those they hadn't killed had gone into hiding, while the passive ones ended up going so passive that few could tell that they were anything other than what they seemed to be in their day job.
When the Second World War broke out, most of the magical girls, both active and passive, refused to fight on the grounds that the pact held that their first and foremost duty was to protect Japan from magical threats, not to serve the military. This actually allowed the Allies to win the war. Once they realized that Japan's magical forces would only respond to direct magical attacks, they pared their own magical forces in the Pacific theater to a skeleton crew and sent the rest to the European theater to confront the mages of the Thule Society fighting for Germany. This turned the war in the Pacific into a purely military exercise.
By the time the war ended, many of the hiding places the monsters had been using had been flattened by bombers who had overshot their target cities and ended up carpet bombing random parts of the landscape. With virtually no monster sightings, much less attacks, the Showa government ended the monster bounty system. With no more way of gaining income from monster hunting and no apparent need to hunt anymore, the active magical girls went passive, and the passive ones left the field entirely. Their descendants, who grew up without ever seeing any of the creatures they were supposed to be sworn to fight against, dismissed the stories as fable and didn't even bother to tell their own children about it.
Three generations have passed since the end of the monster purges. The monsters are slowly rebuilding their numbers. Yet their enemies are now almost gone. Many of the newest generation of the magical girl families have never even met any of their ancestors who last fought to protect Japan against such creatures, much less been trained to do so themselves. And the system that brought about the rise of the original magical girl families has long since been dismantled. How will they respond when they are needed once again?
The origin of the group of people known as magical girls stems from the peculiar thaumic geography of the archipelago known as Japan. Situated on a region where numerous ley-lines intersect with each other in close proximity, the islands are capable of supporting a much higher density of magical creatures than any other area in the world. As humans came to settle the islands, they came into conflict with many of these creatures, some of them because they considered the human settlers to be a food source, and some simply because there was a finite amount of land and resources available and they didn't feel like sharing.
Conflicts between the humans and so-called monsters were generally resolved by massive loss of life among the humans until the 9th Century, when a priestess whose name has been lost to history (But is known poetically as Sakura-hime for the large number of cherry trees that grew in the region in which she lived) discovered how to modify a variety of purification rituals to be able to harm the monsters more effectively than crushing them through sheer weight of numbers. Aided by a local samurai, she was able to rid the entire valley in which they lived of monsters. Sakura-hime was the first example of what is now known as a magical girl.
After securing the valley from monster attacks, Sakura-hime married her companion and trained her daughters and various other girls in the art of fighting monsters. These girls then went out to protect other areas from monster attacks. Once the rulers of these other lands realized that the talent for these monster-hunting techniques could be passed down to later generations, they started rewarding the monster hunters by offering them marriages to their trusted retainers and younger sons, thus ensuring that their lands would produce more monster-hunters to continue protecting the area.
This potential for upward mobility inspired a great many girls to seek out a career in monster hunting. Like the countless young men in western tales that went out on adventure in the hopes of winning a knighthood through some act of valor and thus being able to court a lady or princess, these girls went out hunting monsters in the hopes of winning acclaim and rising from being just another peasant to being the wife of a samurai or lesser daimyo. Those monster-hunters who were uninterested in marriage were often retained by the local lord as a samurai in their own right. Not all of these girls followed the practices invented by Sakura-hime - some tried to invent their own. A great many of these died when their experiments proved ineffective, but those who succeeded started new lines of monster hunters.
While many of the lords hoped that these matches would produce magic-wielding samurai, this was not to be. The time and effort required to learn swordsmanship and magic was such that one could master one or the other, but not both. As the men were needed to fill the daimyos' armies, this resulted the sons being trained to be samurai by their fathers, while the daughters were trained to be monster hunters by their mothers. There were some exceptions - men learning magic and women learning the blade (One daughter of the Aoyama clan who insisted on trying to master both ended up developing a way to perform the rituals used by her family through swordsmanship and thus created the art of Shinmei Ryu), but the general rule was that magic was the province of women, which in turn lead to the coining of the term 'magical girl'.
As the feudal era progressed and more and more areas became relatively secure against monsters, the daimyos thought to use the magical girls to further their own power. At first it was simply a matter of offering the services of their magical girls to their neighbors who didn't have any in exchange for various concessions, but then they came up with the idea of using them as weapons of war. The use of magical girls to supplement the military might of their overlords continued until the 16th century, when every single magical girl aligned with the Azai and Asakura clans were killed by the forces of Nobunaga Oda at the battle of Anegawa. This left the Omi province almost totally defenseless against monster attacks, which resulted in the region being devastated in the years to follow (History would blame the damage on Nobunaga's armies as he continued his campaigns in the Omi province, cementing his reputation as a butcher, despite his being only indirectly responsible).
After Nobunaga's death in 1582, those magical girls who had survived the war came to the conclusion that they had failed in their duty to protect the people against monsters because they were too busy fighting each other. They then forged a pact that they would only fight against monsters, not other men, so that their powers would not be so misused again in the future.
The Meiji Restoration of the 19th Century brought the end of the samurai, and with it the livelihood of most magical girls, who by long tradition had been considered part of the samurai class. Unwilling to form into an army under the Emperor like many of the samurai did for fear of being used in ways that would violate the pact that had been sworn in the Sengoku era, they managed to negotiate a deal with the Meiji government in which they would be paid for the monsters they hunted.
This caused a split in the magical girls. Some turned their traditional calling into a part-time career while they sought other jobs that brought a more regular income to put food on the table when there were no monsters to be hunted. The others went out of their way to destroy any monster they could find in order to bring the bounties in. The scoured the islands looking for monsters, driving many of them near extinction. Some of the monsters treated in this way, such as the Mizuchi, had not posed a threat to humans for centuries, but they were exterminated anyway. By the time of the Great Depression, the active magical girls were running out of monsters to hunt, as those they hadn't killed had gone into hiding, while the passive ones ended up going so passive that few could tell that they were anything other than what they seemed to be in their day job.
When the Second World War broke out, most of the magical girls, both active and passive, refused to fight on the grounds that the pact held that their first and foremost duty was to protect Japan from magical threats, not to serve the military. This actually allowed the Allies to win the war. Once they realized that Japan's magical forces would only respond to direct magical attacks, they pared their own magical forces in the Pacific theater to a skeleton crew and sent the rest to the European theater to confront the mages of the Thule Society fighting for Germany. This turned the war in the Pacific into a purely military exercise.
By the time the war ended, many of the hiding places the monsters had been using had been flattened by bombers who had overshot their target cities and ended up carpet bombing random parts of the landscape. With virtually no monster sightings, much less attacks, the Showa government ended the monster bounty system. With no more way of gaining income from monster hunting and no apparent need to hunt anymore, the active magical girls went passive, and the passive ones left the field entirely. Their descendants, who grew up without ever seeing any of the creatures they were supposed to be sworn to fight against, dismissed the stories as fable and didn't even bother to tell their own children about it.
Three generations have passed since the end of the monster purges. The monsters are slowly rebuilding their numbers. Yet their enemies are now almost gone. Many of the newest generation of the magical girl families have never even met any of their ancestors who last fought to protect Japan against such creatures, much less been trained to do so themselves. And the system that brought about the rise of the original magical girl families has long since been dismantled. How will they respond when they are needed once again?