It's been quite some Time since I last looked in what all Caveman to Cosmos incompasses. Though, I do remember that it's a LOT.
But, to answer your Question:
- adding new Units, Resources, Infrastructure, Wonders...etc. is totally possible, as Mods already have proven. (Though keep in Mind that adding more Assets may trigger the Asset Limit Bug and cause the Game to crash - let's hope for the new Leader Pass to fix that Bug)
- adding new Governments, Policies, Pantheons, City-State Types, and the like is possible.
- adding new Eras,Techs/Civics, and changing the their Connections and Boost requirements is possible.
- tweaking or reworking existing Mechanics to fit the new Changes will depend on the Mechanics and how moddable they are.
- introducing new Mechanics might be possible or impossible. When possible, then it won't be an easy task (as the lack of many Mods that do that have proven), and would definitelly require lua coding (which, if not properly coded, most likely make the Mod Multiplayer incompatible).
- coding the AI to make use of the new Mechanics/Features isn't possible, as we don't have the DLL to make any significant changes to AI. But hardcoding some stuff for AI might be possible in some cases. But again, not an easy task.
So, all in all, it would be possible to make a CTC-like Mod, but it wouldn't be as good as the Civ IV Mod I think (eventhough I haven't played with it tbh and don't know what it all entails, so take that with a grain of Salt).