So if 5th and 4th Grail Wars met, here's what I think would happen:
Iris and Illya drag Saber, Shirou, and Kiritsugu off on some family outing after Shirou offs Ryunosuke. (4th War's Caster Master)
Kariya teams up with Medusa Rider and Emiya Archer to kill Zouken, Shinji, and Tokiomi. And then takes Rin and Sakura out for ice cream.
Iskandar challenges Medusa Rider in a race around the world.