Cu Chulainn would defeat everyone in the fifth war expect Herc and Gil if he wasne't held back by plot and his bad luck.
Unlimited Blade works isn't nearly as much as instant win as most people think it is.
Gilgamesh can hold his own and fight at close range.
We should have more servants interacting in FGO, like have people who absoluty not get along, not get along, and before you say oh their fighting for humanity, there should still be infighting, arguing something, the only glimpse we get of this is Salem with Robin Hood and Sanson and that's it.
The most interesting route from Fate Extra CCC is the Gilgamesh route and we should have gotten a squeal to it, were we see Gilgamesh and Hakuno is space and new problems showing up.
Mordred is a far better written character and more interesting than Morgan and Artoria put together, and desveres more time in FGO that doesn't just focou on her daddy issues.