This is a tricky concept.
While I personally don't like the idea of Mornamath as a Ring Hero (betraying the Witch King?! You bastard!!) I agree it would make the machanism, and the history behind it, all the more interesting and complex. I would say make him the alternate Ring Hero of the faction, keeping the Witch king as main, in parallel with Gondor/Arnor having Gandalf/Boromir or Gandalf/Arvendui; a parallel that makes sense given they kinda all derive from Numenor.
I agree that Mornamath should be the one picking it himself, but like all other Men who picked the Ring (Boromir, Arvendui, Theoden) it should boost his stats, but it also has to have an array of negative outcomes. This counts
especially for the fact that the Witch King is present as well on the field, and I doubt he would take kindly to this betrayal.
Such a mecanic would literally split the Angmar faction, as Mornamath is the leader of the Black Numenoreans, but the real emperor of the whole realm is the Witch King. I daresay that, if Mornamath takes the Ring, he gains a great boost, but like Storm Queen Galadriel, his actions make him a traitor, so units that are not black numenoreans would not help him. I would even say he needs to have a time limit like Boromir, but this time set by the Witch King cursing him.
Overall, i'd say this is a pretty massive thing to make, perhaps not worth all the work that goes behind it.
On the other hand, a Denethor/Malbeth mechanic for the other Ring Hero, the Witch King, might work. Perhaps, once he gains the Ring, he retires to his fortress's sanctum, bestowing powers and terrible magics on the battlefield.
If you think about it, except for the time Glorfindel and Earnur faced him, it's not like the Witch King actually took to the battlefield that often, more dwelling in his fortress and working his sorcery. Granted, Sauron did that far more, but meh...
From the Fortress, he could cast the infamous Everlasting Winter, send out foul spirits to infest a set area of the map (like he did with Barrow Wights), put his blessing on heroes (the old sanctuary of the Witch King power) and even challenge enemy heroes, tricking them to come to him alone (as he did with Earnur, even if that was in Minas Morgul)
This would kinda be a loss, as I love the Witch King to get to the battlefield and make things personal, but nonetheless it's an alternative.
These are just my humble thoughts, dunno what the team has planned, but am looking forward to it, as always!