Well well, first who says this:
Conan The Barbarian - New races in the character creation (hyborian(?), cimmerian etc.) It would be half-fantasy and half-realistic, just like the books and comics.
D&D - Full-fantasy (Dungeons & Dragons), places would be from D&D worlds (Forgotten realms, for an example.) Dragons, orcs, elves and dwarwes, everything fantasy.
Indians & Yankies(?) - Indians: Bows, axes, great combat skills. Yankies: Muskets(?), guns, sabres, lesser combat skills.
Of course, this game is gonna cause an mod-flood when it's finished and there should be pretty good mod-system, so there would be no troubles with different kind of mods, you could use only one mod per character.