Lets get some things back from civ 2 and some few more things

[Corvex]

There should be some limited kind of terraforming, but it should only come very late in the game, with advanced technologies. They need to bring back theocracy/fundamentalism as well, but they could handicap it by making it's science abysmal.
I also think that they should include some kind of trade/commerce unit; that way you can build a commercial empire, and you won't have to keep building military units until you have to invade someone.

 

[dh_epic]

Abysmal science didn't really cut it in Civ 2 when it came to balancing Fundamentalism. I'm not sure lowering it from 50% to even 10% would make much difference. A lot of the time, you'd move to fundamentalism, drop your science rate to zero, and use your heaps of cash to buy techs, or steal them with espionage... or as the case was in Civ 2, to acquire techs by conquering more "advanced" civilizations.

I think the cheap/free/effective units were a bit too unbalancing. Maybe if other nations had certain benefits of that kind. E.g.: Democracy gets a great defensive militia.

 

[CurtSibling]

I reckon there is much in CIV2 that is worth replicating and building on.

Hence, why CIV3 Conquests couldn't help introducing many traditional CIV aspects!

Because they worked!