As far as I can find, this was last asked in 2014 and never got a reply, so:
Defeated factions coming back after a certain amount of time/conditions are met.
This would really benefit players like myself who play very slowly, and only become ready to start their own kingdom once one or two of the factions have already dominated the map. This would also allow one game to be played indefinitely, since you can just speed up time for X months and come back to a whole new map.
Idea for how they could come back: The king and X amount of exiled from Pendor lords all spawn near the king's original capital and lay siege. If they win, new faction, if they don't, reset timer/conditions.
My bad if this has been asked before, I could only find one post about it (https://forums.taleworlds.com/index.php/topic,268612.msg7478459/topicseen.html#msg7478459).