I actually really like Chiska's original proposal, but it needs something to spice it up.
So as far as I can tell, Mornamarth only serves because he has plans to rule, and similarly Durmarth is an extremely deadly and powerful warrior in his own right. So actually, what if these 2 as well as the Witch King could ALL take the Ring, with differing effects?
Witch King: If the Witch King takes the One Ring, Sauron becomes recruitable at the Citadel in his Ring form, and the Witch King permanently gains the Ring buffs he had formerly. If Sauron dies he drops the Ring, and Witch King also dies and loses his powers.
I would also like to note that I absolutely hate the idea of the One Ring disappearing from the game, I have hated this feature of Lothlorien ever since they released and have tried to go against it. It is completely unfair for the other player, they have no chance of killing the Ring Hero and regaining it if Galadriel denies the Ring, and Galadriel permanently gains all those buffs and the ability to get Frodo and Sam, who are both powerful heroes in their own right.
Mornamarth: If Mornamarth gains the One Ring, he becomes the new ruler of Angmar. The Witch King dies and cannot be revived until Mornamarth loses the One Ring, and Mornamarth puts on the Witch Kings armor and weaponry to rule Angmar himself. He could have a 2 palantir system featuring enhanced versions of both his and the Witch Kings abilities.
Durmarth: If Durmarth gains the One Ring, he becomes an out of control unstoppable warrior, whom even the Witch King himself dare not raise his hand against. He could keep his two palantir system, except now 1 palantir could act as a mass slayer set of abilities, while the other focuses on hero killing, to highlight the fact that he has become such an unbelievably powerful warrior.
I think giving these options would be really fun and give Angmar a lot of diversity with their Ring Heroes just like they have a lot of diversity as the foundation of their faction(or, at least they would if Dire Wolves weren't so OP D:) I know this is an old topic, but lets discuss this further, I think it has a lot of merit and I should have gotten into it sooner.