Nanoha crossovers eh. I think its a series that's actually quite compatible for crossovers. There are a lot of different angles I can see for Nanoha x-overs depending on the universe. Unregistered and potentially dangerous artifacts, the discovery of people or groups of people with magic in other worlds (also the existing political position of those groups, what their policies are towards the non-magic/magically ignorant population) or people with abilities that might be dangerous to the other dimensions and so on.
Something I often think about with the Nanoha series is that most of the magic that the main characters use is largely combat oriented and their magic is broken up into very specific areas, mainly attack and defensive abilities. There is not a whole lot of variety of different effect type magics, instead the magic is focused largely into being more for that one specific area. In my opinion this would leave the TSAB and Midchilda mages more vulnerable to more obscure and unusual effect abilities (ie stuff like sleep spells, dimensional/space time distortion, illusions, mind reading, area and environment altering spells and a bunch of other things not oriented towards direct attack spells). Even their supportive spells are based mostly on healing or binding from what I saw in the series, which isn't that big a variety. There might be some more but probably not commonly used in combative situations or alternative confrontation means (ie ambush, indirectly manipulating people, espionage and information gathering).
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Potential crossover series:
Mahou Sensei Negima -
The series has a vastly different type of magic system and it would be challenge to find ways for people from Negima to match or get around the sheer amount of firepower that the Midchilda style magic has. Some of the insanely strong people like Evangeline and Rakan (and later maybe Negi and Kotaro) could potentially match that kind of firepower, but in the early stages it would definitely be hard. Negima does have a much larger variety of abilities especially once half the 2-A/3-A class gets pactios. That could lead to very inventive use of the non combat oriented magic and pactio abilities in case of a potential confrontation. Asuna's magic cancel could be the worst nightmare of Midchilda mages (imagine Starlight Breaker being fired and fizzling out before it hits Asuna). The combat environment would also be quite a bit different from what the Midchilda mages would be used to in the normal world and they'd have to be less flashy and more covert in some scenarios.
The political system of the magic groups on earth would probably be a surprise for the TSAB. There is a very well established system of local groups over seeing affairs of the magic community within certain regions and it is quite interwoven into the existing world and ordinary politics. If the author chooses to make it be the same earth Nanoha is from it would be a bigger surprise for the TSAB considering that they've been monitoring earth for a long time but never discovered the magic community. To top it off the existence of the world of magic interconnected to earth would be an even larger shock. It would definitely raise some questions as to how much authority the TSAB could exert when there are groups aware of the supernatural that have a job to deal with it already. While the TSAB limited their interference on earth to a minimum their main reason for intervening might be nullified if they discover this since they previously believed there was no one even remotely able to handle such situations on the planet.
I can easily imagine an assignment given to a young TSAB agent (say Teana or Subaru) that entails getting into Mahora or another magic hot spot to observe and learn about the existing magic styles on earth and what the magic community is like. Of course they get caught up in the mess with Negi in some way or another, whether or not their cover is entirely blown is a different story though, it could go either way.
There was one fic started for Negima and Nanoha
http://www.fanfiction.net/s/4423680/1/Maho...l_Nanoha_Negima (hasn't been updated since January though). I found it to be decent.
xxxHolic and Tsubasa Chronicle -
Yuuko's style of magic is quite different from the Midchildan style obviously. I can see Yuuko attracting negative attention from the TSAB for a number of reasons. The side effects of her magic are often not very good and cause problems for people though often on a fairly small scale they can be quite bad in some scenarios. She also seems to have problems with explaining the exact specifics of what her magic will do to some of the more random people who enter her shop and the hitsuzen and exact compensation arguments probably wouldn't cut it for the TSAB. She also has a number of potentially dangerous magic artifacts in her possession and has freely handed some of these artifacts out to people who really don't know what they are getting into. Again the hitsuzen reason really doesn't cut it when she knowingly gives out dangerous artifacts to people who don't know the consequences of using them.
The biggest reason Yuuko would warrant the TSAB's attention is that she has the ability to send people through dimensions. The TSAB likely has no method that would allow them to track the people she allows to travel dimensions and regulating dimensional travel and watching for things that could cause dimensional instability is one of their primary purposes. The TSAB might not technically have the right to try and regulate dimensional travel and oversee inter dimensional issues outside of dimensions and planets affiliated with them, but they'll probably do it anyways.
The whole issue of dimensional travel could cause trouble for the Tsubasa Chronicle group if the TSAB gets wind of them. The problems caused by Sakura's feathers would also warrant their attention. The feathers are items of unknown origin with a massive amount of power, can partially alter reality and are scattered throughout several dimensions. It certainly seems like something the TSAB would act on.
Ranma 1/2 -
The sheer number of curses, magical artifacts and supernatural things around Nerima might warrant an investigation if the TSAB discovered it. In a world where magic is supposed to be rare if not nearly non existent a place with such a large concentration of supernatural events and items would be worth checking out.
Discovering the level of some of the martial artists in the area would be a new find for the TSAB if such a thing was previously outside their knowledge. The usage of ki might also raise a few questions since they are used to magic based powers and have only seen technological based weapons outside of that. I can envision some of the TSAB higher ups deciding that a person with Ranma's abilities and level of martial arts training could be useful for their operatives. Having infiltration agents that can fight like that without the need for magic devices could be incredibly useful for the TSAB. The agents would have a chance at fighting back if need be without having access to a magical device and it would massively decrease the chances of their agents being detected if it was common to check for magic devices and where said devices came from. Somehow it ends in Ranma getting a potential job offer as a combat instructor for the TSAB.
Seen a few one's so far:
Second Night: Reinforce Drei
http://www.fanfiction.net/s/4703969/1/Seco..._Reinforce_Drei
Ririinkaru Ranma
http://www.fanfiction.net/s/4967721/1/Ririkaru_Ranma
Magical Lyrical Nanoha 1/2?! (on this forum)
http://z14.invisionfree.com/The_Fanfiction...showtopic=14523
To Aru Majutsu no Index and To Aru Kagaku no Railgun -
I think the advancement of the esper abilities taking place in Academy City could attract the TSAB's attention. The abilities are quite a different type than any system of magic and the rate of advancement of the esper abilities has been fairly rapid there. It would be another type of infiltration and observation mission where someone from the TSAB is to take up residence in Academy City to observe the esper abilities and try to uncover the nature of the Brain Development program. The TSAB would be interested in learning how these abilities are being developed, what are the eventual aims of the groups behind Academy City and the research being conducted there are.
Numerous complications would come up consider several factors, such as the ethics of the research being conducted (ie experimenting on children for some of the earliest experiments in the brain development program many of whom died and the Sisters experiment with Misaka's DNA) and if the TSAB should become involved given the nature of such a situation. Index would be an issue, given the nature of how the TSAB deal with Lost Logia and dangerous artifacts it would be hard to ignore a person with thousands of grimwoires filled with dangerous spells in her head. Given the recent ethics reform after Strikers though they'd have to take special attention to the fact that Index is in fact a human being and not just another artifact. Toma's ability to nullify any type of magic ability might become a concern or an asset. They could potentially approach him and explain some of the situation once his troubles with Index start. The TSAB could then cut a deal by guaranteeing Index's safety in the event that something happens which Toma can't handle (the other benefit for the TSAB would be that it is also a convenient excuse to keep watch on Index while not denying her freedom). Other things to pay attention to would include some of the top espers like Misaka and Accelerator, The Judgement as a relatively independent group with access to some slightly more confidential information and a convenient way to learn what to expect when facing people with esper abilities and the reason why the mages of that world are in conflict with espers despite that most espers don't know they exist.
Kaze no Stigma -
Another series with magic users existing in it. The main purpose of mages there is for the exorcising/extermination of evil spirits, demons, monsters etc. Most of the magic users in this universe are based around using an element of the four classic wind, water, fire and earth. The majority or the higher up government officials are aware of the existence of the supernatural and there is also a special division of the police force dedicated to keeping the supernatural under wraps, capturing criminals using magic or other supernatural abilities and also taking care of other issues related to magic users. This division though seems fairly new, is drastically under staffed and also seems to lack any powerful combat oriented magic users so they constantly have to ask magic clans or freelance mages for help. There are exorcist clans which have existed before the police force and gotten very rich from their reputations and monopolies on that business. There is a guild mentioned which gives freelance mages/exorcists jobs but there is no real reference to them outside of this. There are some magic users in this series which could actually be a potential match for even the strongest mages in the Nanoha series. The series doesn't show a massive amount of interaction between the clans, freelance mages and police force only a couple of key people that fall in each category so it would take a bit of imagination to figure out how to work the dynamics of that all into a story but its doable.
Pokemon -
Nanoha getting dimensionally displaced somehow, ends up in the Pokemon world and has no immediate way to get back. In the mean time until someone from Midchilda locates her she decides explore this new world and learn about the creatures called pokemon which inhabit it and the people who catch and train them. Could wind up with her catching a few pokemon and deciding to become a trainer in the meantime.
This idea would be more of a cute and fun one rather than a serious one. I'd find it somewhat amusing. Also imagine Team Rocket being blasted off via Divine Buster or Starlight Breaker.
Mai Otome (anime) -
Largely because I'd find it interesting to see a fight between an Otome and higher level Midchilda mage. Both sides have some rather high levels of firepower. This might also inspire a more high level investigation as to the technical side of how Otome's power works and if there are any similarities between the GEMs and the Midchilda based magic devices. The TSAB's appearance might become an issue since the TSAB is an entirely unknown element in a world used to relying on a fragile system of power balances with the Otome in each country as a replacement for war (granted they are making some progress after the mess with Nagi and Artai.
Mai Hime (anime) -
The TSAB mages could potentially upset the current HiME "carnival" and cause problems for SEARS and Nagi (plus the Obsidian lord later on). Hopefully they'd try to talk sense into more of the HiME before they go and try to kill each other off. If not they could probably try and take them down via force. The school situations and other random interactions between the characters of the two series would likely be interesting enough on their own as well.
(there is one cross between these two started
http://www.fanfiction.net/s/5023003/1/Collision but it's only at chapter 1)
As a bonus for anyone who wants it there is the first chapter of a Nanoha and Full Metal Panic crossover
http://www.fanfiction.net/s/4380014/1/Full_Lyrical_Panic_As