Okay, I had this idea for the MMO version:
How about, instead of moving your territory every time it's conquered, you become part of the victor's "Empire"? That way you would have to pay a regular tribute to that player (A certain % of income?) and have some of his soldiers patrolling your base and generally bullying your citizens. He could also absorb your remaining soldiers into his army and start conscripting your citizens.
This would not be the end for you, because your surviving heroes could go into hiding (turning the game into more of an RPG) and form an underground resistance movement, hopefully and eventually liberating your base or evacuating your loyal citizens to settle elsewhere.
This would make it so powerful players have to not only protect their borders, but also keep a sufficient garrison in their occupied towns in order to keep control of the population.
Anyways, hope you like my ideas and I do admit that they are a bit rough, but I do hope something similar could be incorporated into the game.