Not sure where to put this as it kind of has something to do with both Skyrim and Morrowind.

Red Mountain blew its load....anyway. We go to Solstheim to fight Miraak and we come across Ash Spawn and undead that had been at the mountain.

Is it possible the Nerevarine was not able to kill Dagoth Ur as only dumping him and the heart off into the bottom of the volcano?

I mean, Dagoth Ur did still have his divinity as a god. So, its probably likely he was able to take the heart, as he was drawing power from it.

Is it likely he's still very much alive and with Lorkhan's divinity?

Several NPCs, including the Ashlander wise women, caution the Nerevarine that this isn't the first time (s)he defeated Dagoth Ur and thought him dead. So who knows?

Kagrenac's enchantment on the heart was the conduit which allowed the Tribunal and Dagoth to access its power. Though the Tribunal apparently defeated Dagoth numerous times, they could never bring themselves to sever this conduit. That's what allowed Dagoth to come back each time.

While anything's possible, I think a "zombie Dagoth" would dilute the Nerevarine's victory in TES3, and it's probably not a road they would want to go down.

I think the return of Dagoth Ur could potentially be done brilliantly or horribly, but I don't have confidence in Bethesda's ability to do it brilliantly and there's really no in between.

If I understand correctly, Dagoth Ur was cut off from the Heart of Lorkhan, hence why he was able to be killed and not simply come back to life again.

Regarding the Ash Spawn, the original ash creatures in Morrowind, as I understand them, were in fact Dunmer followers of Dagoth Ur mutated by Corprus Disease; they could talk, and would I believe could progress through stages (Ash Ghouls appeared to be progressions from Ash Zombies, for example) whereas Ash Spawn are literally made of ash.

Since Vivec was not entirely powerless after the severing of the connection, but severely weakened, it's possible that Dagoth Ur survived his mortal destruction, but it would really put a damper on the powers of any future reincarnation. Obviously, returning to life in an active volcano wouldn't be a pleasant experience, either. The return of Dagoth Ur as a mere mortal, or a mere shade above, would hardly be a Nirn-shaking event, even if it would be possible.