[Maraxus]

Is this a "Dom4" kind of question?

I could think of some ways.
Force the players to build up "Support networks" in their provinces, whatever this means. So, in order to have decent support/income/resources in his provinces, some provinces need to be knotes, that - should they fall - be a fatal blow to the whole region.

Likewise - the home province - the capitol: In the early game capturing someone's capitol seriously hurts this player. Later on it's no major thing. That should be diffrent, imho. Think of the Civilisation games: You have one capitol - you can change this for some ressources and time, but if you are caught without one, you you will have economical (and maybe moral) problems untill you reconquer it or set up another one.

And last, large armys are not useful in late game, which could be altered: Make large armys able to conquer/control more then just a single province. Alter the way remote army attack spells like "fires from afar" work, to allow protection against it (like protective mages).