It all depends on who, exactly, makes up the other side(s) - I'm currently thinking a four-way team battle, each based in one of the house dorms.
Actually, I was kind of thinking of this Servant for Shirou.I'm assuming Shirou summoned Arturia again, since his only other possible summon EMIYA is already there.
Well, we already have a Lancer . . .Who else, according to CCC Karna was summoned even if you never see him in Vanilla Extra.
Though he could show up on one of the opposite sides.
Oops - I'll fix it when I get a chance. I am more used to hunting pics of Artoria as Santa Alter, so I think my brain defaulted, as I was in a rush to get the snippet out before I left town. Sorry.