Caveman2Cosmos is the most developed (and largest) mod for Civ IV,
(more content than all others in Civfanatics combined and simply a Dream Team of modders pushing the limits of what Civ can do.)
I highly recommend you play a few deep games since C2C seeks to improve the whole Civ series.
Every version is like a whole new Civ Expansion, and the improvements are astounding.
You really should keep track of the amazing progress in the C2C project forums, even just to see how much improves.
-They are even looking to create alternate timelines and integrate the whole Final Frontier mod into a whole multi-map 4x space and Galactic Future expansion.
-They've done everything from completely rewrite the AI and turn times, completely expand the resource limit, pioneer whole new sets of major features, work on bypassing the game engine to become multi-core, to adding living world and simulation features, resource and equipment chains, reworked culture, religion, and civics, to exploring advanced features like a geo-realistic climate system, advanced events, and reworked user interfaces and game options.
C2C is like a whole new Civ game with the promise of being better than Civ 6.
To see for yourself, check out:
Civ4 - Caveman 2 Cosmos - Project forum
http://forums.civfanatics.com/forumdisplay.php?f=449
and
C2C-Idea Inspiration and Organization
http://forums.civfanatics.com/showthread.php?t=464235
for a semi-updated list of some of the major ideas being explored.
Once you really play it, I bet you will be surprised.