So, the character of Shirou Emiya has a lot of potential on his own to create a wonderful story out of this idea, but I believe Archer is even better for this idea. The reason I believe this is because Archer has the dilemma of whether he can even be a hero again, though since this will be post True UBW Route Archer, he will likely have an easier time readjusting himself to being a hero than otherwise. Shirou Emiya, or Archer, is, as they say, a character that was born in the wrong era, so what happens if he does live in that era? It's not the exact same as if we lived out his entire live there not ever having lived through UBW, which would be an interesting story in and of itself, but it is a similar idea. The method he would get there would probably be Zelretch, and his contract with Alaya would be terminated. Archer can meet several legendary heroes along the way, go on multiple journeys, pick up new weapons and skills, and maybe even meet gods. I think this could go a long ways if approached properly. Since I think that this could branch out in multiple ways that I'd enjoy, I won't ask of any other specifications other than one more. Remember to scale Archer properly; don't take away from his capabilities. He can win against a berserker Heracules 6 times or so before he dies, assuming he uses his reality marble, but even he would struggle with mythical creatures, gods, and Magus with far higher quality and quantity of magic circuits. Against heroes themselves, however, he'll probably be able to take most of them on in a good fight even before he gains any extra powers. Winning said battles is a different matter, of course.