The unit would be AI controlled and would be controlled by the Dagoth-Ur faction that would appear then, essentially a boss fight. Think of it like the Mongols in Vanilla just that instead of 100.000 units, you get one almost invincible unit.
As for the the artifacts. Well the Daedric religions should not be limited to artifacts, they should have flavors, holy sites, and specific goals like all other religions. Getting the artifact easily is the bonus for doing it in an exclusive faction.
So for instance, getting Dawnbreaker as a Pyandonean might require 3000 favor, and 1500 prestige instead of the much cheaper alternative when using the Cult of Meridia faction.
And finally...About the Chimers..
Because if a Chimer returns to Morrowind, converts to Almsivi, and eventually becomes the immortal queen of Resdayn, like in my game, it's pretty stupid she can't make the Ebonheart pact.
@Chimer: so we should open the Pact to all races basically. Since the immortal queen of Resdayn can become anyone. All you need to do is have strategy! Heh, I was Orsimer King of Morrowind ^^, before I gave it to my son and went about conquering Black Marsh. So it would also be stupid for my Immortal Mage Orsimer King of Resdayn not to form the Ebonheart Pact. Or my Nord King of High Rock form Daggerfall Covnenant. Again... Dunmer =/= Chimer. They are not longer the same sub-race. They are both Mer, but not the same sub-race, aka Chimer. Dunmers are Dunmers, and Chimers are Chimer, just like Nords are Nords and Bretons are Bretons, despite Bretons being a branch of the Nords. Just as Reachmen =/= Nords =/= Bretons.
Your Immortal Chimer Queen of Resdayn is like any other Immortal character of any Race holding the crown of Morrowind. The fact that she is Chimer makes her no more special than a Tsaesci, Kamal, Argonian, Khajiit, Bosmer, Altmer or Ayleid. So if you want we could discuss opening the Alliances to anyone basically. Eliminating what made them special in the first place.
@Artifacts: Hmm, not convinced. There already plenty of achievable artifacts, we like that he Daedric ones are a specific reward for staying true to one Daedric Prince. Because in all honesty, this can go further. Even the Eight Divines believe in the Daedra, is the nature of TES. Everyone knows that the Daedra exist, so by acknowledging their existence that implies at least a limited form of belief. So everyone should be able to get those Artifacts. Which leads to the Daedric artifacts loosing their special meaning and just becoming another mundane named artifact.
@Dagoth: I don't think we can add him as a unit nor as a character. Apparently he himself is a God, and the only reason he was defeated was that his connection to the Heart of Lorkhain was broken, making him a mortal.
http://www.uesp.net/wiki/Lore
agoth_Ur. Which means that outside of an Event where the player has to do that, before fighting him, a military unit Dagoth-Ur would be invincible, unless fought by another God. So... no.