You know, the future of this could get dark really quick, because it looks like it's set before Fate/Zero (as Sakura was already given to the Matous by then). As a result, you'd wind up with Kotomine going and attempting to kill two little girls, Archer fighting his father and first love in an attempt at protecting what he thinks of as his daughters, the girls wondering why he's fighting people he has such happy memories of, and things generally going FUBAR.
Gilgamesh might not be summoned, though; I'm not sure if that's a good thing or a bad thing. Thinking on it, though, he might be summoned as Berserker, because the Master of Lancelot-Berserker only entered the war in an attempt at protecting Sakura, Needless to say, this would make him dramatically weaker than his normal state.
I wouldn't be surprised if Archer pulls out Avalon in an attempt at saving the life of Rin and/or Sakura after they get almost killed, though.