Here is the link to the main page of the Caveman 2 Cosmos mod: http://forums.civfanatics.com/forumdisplay.php?f=449
And I posted my first impressions here: http://forums.civfanatics.com/showthread.php?t=488885
Keep in mind...FIRST IMPRESSIONS. Not a review or anything like that and it was only 3 hours in.
They want to add multi-maps, so you start on the planet and as you advance technology...you'll (and the AI) will be able to colonize the moon and then expand into space itself. This of course is a very hard thing to accomplish (multi-maps), and it is about another year away at the very least. But the mod itself is actually rather finished and has so much content. Currently it is most popular mod out, but it came out around the time Civilization 5 was released and there was a lot of uncertainty about if all the work would be worth it or not. But, a lot of Civ 4 fans went back to...well...Civ 4 and the mod has actually been quite a success.
So I'll sum up the features, but you'll find way more (and better) on the forums
-Constantly updated (especially SVN version, gets 1-4 updates a day it seems)
-Huge amount of research
-Lots of new civilizations (though the civ itself only has unique LOOKING units, they don't get actual unique units. The leaders pretty much make the civ unique)
-Big improvement to espionage
-Big improvement to diplomacy
-A lot of new settings when creating a game
-Your map can actually change over time (someone had a map and he somehow got a massive problem of global warming, and his map turned into a Dune style map...all desert)
-A lot of new ages to go through
-New map size all the way to Gigantic (hope you have a powerful PC :P)
and...well there is a lot more. But, since I'm actually still sorta new to the mod...I can't really think of them. You'll find a lot more info on the mod's forum.