[Agrajag]

How about having different population types for the different nations?
When one nation conquers a province, the local population is slowly replaced by that conquerors.
The special population will only be able to produce gold/resources for the corresponding race.
With the new populations, you could give the death themes a population changing effect, rather than just killing, so a province deep in AE dominion will have all of its population replaced by useless undead that don't produce gold for them, this way the effect of the death themes wouldn't be so deadly, and provinces that were once under their effect can be reestablished, and become useful once again.
A potential spin-off would be giving special properties to different population types.
Pangean population might produce extra supply, and perhaps CW-Undead population will produce less gold (instead of no gold as it is right now) which will help to make CW more fun.