Drakarska
Epic Dadness
SMAC or an official FFH would be ideal.
SMAC was and continues to be awesome.
Same with FFH, and given market reception of WoW and similar titles, there's definitely money to be made in fantasy settings.
My main negative about FFH (mostly about its children, Fall Further etc.) is that they never got a stable codebase. You have to come up with a plan of functionality, design for game balance, implement it, bugfix, and be done. When you constantly tweak the core functionality and constantly add more and more elements, you never get stable and never get balanced gameplay. As a result, I had a significant bad taste over what should have been an excellent mod. By making it an official spin-off, I have no doubt Firaxis would hit these criteria (at the cost of being less feature-filled, but that's the trade off).
Just a little info for you. Kael did fix a lot of issues in FFH 2, and some really talented modders took that a step further with some modmods. Y ou may want to take a peek over in the Civ IV mod threads.