I agree that the mid/late game can get repetitive.

Rebellions are part of the answer but as mentioned the main reason people dont finish a campaign once the "critical mass" is reached is that it is no longer fun.

I think it just needs some variety in there, some random thoughts:

Interesting late game units that take a lot of teching up to reach.
Rebellions/Faction reemergence (decent sized ones not the RTW type)
An expansion on the jihad/crusade/pope situation which may change late game??
Unlock units/attributes/factions after a campaign is completed.
Have an epic high quality end game clip when total domination is reached!