I think a post-apocolyptic FPS would be fun... a mix of all different types of weapons from different ages... mutants and such.
Also I think there should be a nowadays game. You could use tomahawks, battleships, aircraft carriers, chemical weapons, nuclear weapons, napalm, etc... might be hard to carry out, but would be fun... imagine an Gulf War 1 MMORPG/FPS like planetside... (and no I'm not thinking I'm being very politically correct for the current environment, but hey... the war will last for years, it's time to forget about it.)