Hitler as a "heroic spirit" makes more sense than I'm sure people are going to give it credit for, at least in the context of the nasuverse. There are plenty of "heroic" servants that are far from the traditional definition of "heroic."
I really liked some of the servants in Fate/Extra and felt that they fit very well (Robin Hood, for example). Also, Nero was fantastic, and not just because she was voiced by Sakura Tange (though it didn't hurt). To that end, I think maybe they could explore the Greek mythos a bit more. There were some really interesting stories that came from Greek mythology, my favorites being Odysseus' return from the Trojan War as depicted in the Odyssey (and all the associated anecdotes) and Orpheus' (ultimately tragic) journey into the underworld to save his wife. There's a lot of stuff there to work with.
I also liked the "Divina Commedia" and the associated characters, though there's a lot of historical political context in the tale. Still, at its face, it's not hard to understand and it's a widely known story, so maybe Nasu could do something with that.
They could also do some interesting stuff with modern history, such as figures from the past hundred years. Revolutionaries, despots, thinkers, activists, there are a lot of different types of characters to choose from. Or maybe a few hundred years back: the discovery of the new world, colonization, the Renaissance, British imperialism, and all of the things that led to the modern world?
There's a lot of material in history that can be leveraged to spin all sorts of tales, I think. I like the idea of using historical figures to tell a story because you already have interesting stories to work with, characters with all sorts of motivations and ideals. There's a lot you can do with it and the fact that we don't really know all that much for certain makes it even easier (revisionist history is a field unto itself). And of course, since it's fiction, you can always twist things a bit to make it a bit more interesting.
VNs are superior to anime, don't @ me