I didn't see a suggestion thread so here some things I thought about during play:
Burned EarthEach city, castle and village you conquer is destroyed and either removed from the map or replaced by a ruin. As you remove anything valuable you get more loot and money than normal in Native - without the income from fiefs you might need another way to get money. Instead of getting the loot directly you could place it into the ruin.
Oh, the corpses you could get from killing a whole city.

- Variant: You can choose to destroy it when you conquer it (allowing you to keep some choice holdings).
- Variant: You can choose to destroy it by talking to the blood fountain, sacrificing all the lives in the fief to it.
- Variant: Destroying a city or village results in wandering groups of undead (hostile or recruitable)
Undead KingdomEach city and village you conquer is changed to undead (villagers, peasants, guards...) but otherwise stays the same.
- Variant: When the other kingdoms reconquer it they send new settlers there.
- Variant: When the other kingdoms reconquer it they raze it.
P.S. I prefer Burned Earth
Undead LordsYou can sacrifice captured lords to the blood fountain creating a wandering horde of zombies under his command. This could result either in a undead copy of the former lord or just a generic death knight.
- Variant Hostile: The new troops are hostile to you and anyone else.
- Variant Neutral: They patrol an area and are neutral to you but hostile to all others.
- Variant Ally: Upon creation you can send them to patrol an area or attack a lord.
PrisonersIs it possible that prisoners can be handled based on the capturer.
- Undead/Necromancer capture Undead - fast recruitment into own ranks.
- Undead/Necromancer capture Living/Necromancer - killing and recruitment as undead into own ranks.
- Living capture Undead/Necromancer - destroying prisoners.
- Living capture Living - as usual
For the player:
Undead or summoned creatures can be easily recruited by talking to them without morale loss.