I have a lot of fun when playing mortal empires. But I almost never finish a game. one of the main reason for this is, that most factions have very, very extensive victory conditions, even on the small victory conditions.
If I remember correctly even for the small Grimgor victory I need to invade naggaroth or have military allies there. These conditions are anything but short and let the campagin drag on and on for quite a while.
Thus I suggest to implement something akin to the homeric vicotries in Total War Troy. Each Legendary Lord has his own goals to complete. These would consist of completing all quest battles and reaching certain goals on the campaign map.
For a small example on how these victory conditions might diverge between subfactions:
For example for Karl Franz the goal could be to control all empire provinces, have the reiksguard castle and the imperial palace built, have a certain number of generals on level tweenty and keep archaeon in a wounded state. This emphasis on him as the protector of the empire.
Balthasar Gelt on the other hand must also control all empire provinces and keep archaeon in a wounded state, but in addition he must construct the colleges of magic and research a certain number of technologies and earn a certain sum of money per turn.
This character victory could work as an third additional victory condition to the short and long victory condition, allowing payers who want to enjoy the mortal empires campaign for a short while, to acutally win it. Completionist could then of course try to work on short and long victories.