If it's a standard holy Grail war then
Saber:Mordred, she's intresting, powerful and could be fun to explore her that doesn't involve her daddy issues.
Archer:Gilgamesh, no reason needed other than he's the king
Lancer:Bhima. one of the strongest people in the Mahabarata and is one of the half brothers of Karna and Arjuna, and would fun to see him outside of Ordeal call one.
Rider:Ozymandiyas, he's basically Egtption Gilgamesh and I want the two of them to cause problems
Caster:Nzambi, I just want to see her outside of Requim, she is an Earth Mother Goddess after all.
Assassin:Agravain, we know he qualifies for the class, and what better way to see him get his ass kicked.
Berserker:Samson, Samurai Remnant speaks for itself.
Ruler:Uesugi Kenshin or if not her then Astraea.