Inaba said:
He tears asunder the old, rotting structure of the corrupt and burns the debris to nothing so that the survivors might build new and sturdier things on purified ground. He is the firebrand of revolution, the harbinger of long overdue change, the spark that ignites the already piled up tinder and the wind that fans the flame. He is Surtr's burning sword, the all-consuming flame that rises up to consume gods and giants at the end of days along with their petty grudges so that a new world may come about, better and more innocent than the old.
Or, for a less nuanced alternative, base him on the quote, 'Let Justice be done, though the world burns'. Base him on the explicit reversal of the modern ideal, so that he espouses the principle that, 'Better a hundred innocents should rot in prison than even one guilty man go free'.
That is extremely dark and completely divergent from canon.
I like it.
Not sure how feasible it would be and whether it would lend itself well to a plot, but I like the idea. Now, how to implement it...
Shirou, upon being rescued and learning of the Holy Grail War and the destruction it has caused, decides that it's more important that the people who caused the tragedy never have a second chance to do so? If he's picked up by someone Anti-Association, he could be raised to believe that the MA must be destroyed to create something better. Even if someone points out that more peaceful changes could be implemented, the violent upheaval and Rebirth origin leaves Shirou torn. Violent upheaval now to make way for the new, or wait for later? After all, people will embrace the new change most readily when the corruption is most apparent.
No, I'm not considering the 'MA is good let it be'. I like the Rebirth Origin and feel that Shirou would never stop wanting to see something new born. The best he would agree to is that perhaps it should come later. In my mind at least, I'm willing to be convinced otherwise.
Complimentary idea: Shirou wants to see a perfect creation, a utopia born from the ashes of the old. But he realizes that if such a utopia came to exist, he would feel the urge to tear it down and watch it reborn anew, even if the new creation lacked the utopia's perfection.
Shirou joining the HGW...he wants to use the Grail to achieve his goal, or he wants to use the HGW as bait to kill people associated with the HGW. If he becomes a Master by accident, he'd have to plan on the fly more and that's probably a good thing.
I'd suggest that an aggressive Shirou pull a Kotomine and ambush a Master for their Command Seals, possibly Kotomine himself because I would be amused by a Gilgamesh and Shirou team up, but I think you need to have been chosen by the Holy Grail in order to be a Master.
Alternatives....Shirou steals Shinji's Book of False Attendent. Temporary measure at best unless Shirou can prevent its destruction, but it's there.
Hmm....Shirou is a Master, makes plans in advance, and assassinates corrupt Masters for their Command Seals to get a second Servant and order them to run suicide missions. Shirou and a True Assassin? Part of me is still trying to shove in a Shirou and Gilgamesh team up. Shirou couldn't force Gilgamesh to do anything, but their goals are similar and if caught in the right mood, Gilgamesh may indulge Shirou. If The King puts that A+ Charisma to use he may inspire Shirou to solidify his Reality Marble.
Now, keeping Gil from finding some new Master and coming back for Shirou when the whim strikes, as opposed to teaming up with Shirou? That's still a possibility. Also, I wouldn't rule out the possibility of Shirou deciding that Gilgamesh is another one of those people that needs to go.
Hmm, there's a limiter on Shirou. He knows of Gilgamesh but lacks the power to actually kill him, so he puts his plans to murder his way through the participants and creators of the HGW on a back burner to find someone who can kill the King of Heroes?
Honestly, selling Shirou forming his own RM without his Heroic Spirit counterpart running around is difficult. The best I got is a Heroic Spirit seeing the beginnings of an RM within him and molding him into the proper mindset to accept its existence and use it.