I'd prefer Firaxis stick with historical settings, even in a spinoff. The market is flooded with space 4X games (Endless Space and its sequel, Galactic Civilizations, Stars in Shadow, Stellaris, and many others), most of which are probably better than what Firaxis could come up with in the genre. If we do get a spinoff, I'd rather see a bronze age, mythological, or even neolithic focused game over an original setting.