I would love to play (mod) civ 2 unlimited version. If I can play a civ 2 (same everything, using original civ 2 assets) game with this clone then I'm interested.
Unlimited units types, city styles, techs, tile types, improvements, resources, map size, wonders, buildings, governments would be first great feature. So probably support of multiple file types (original, extended, and than of better quality, 3D?)
Then customization of everything (like settler cost of 2 pop) is another one.
Third feature would be things introduced in civ 3, 4, 5, or call to power and possibility to use them or not. More control/significance of resources, other corruption/waste models, civ specific units/unit types, buildings, small and great wonders - there are tons of things that could be impleneted, and probably every person would like some of them (and another person some others). But I think it wouldnt be to hard to just write them all (things already present in civ games, then things that aren't) and see whats possible to do.
I could help with that.