I thought Luagar2's theory on Dagoth Ur was that Dagoth Ur no longer required the heart as the vessel to immortality and that he could now 'dream he is alive' or something and is just like Lorkhan, a god thought dead by all but who can manifest an avatar of himself.
The theory was more on the nature of Dagoth Ur in general than what he can or cannot do now. I did make some speculation though. He was dead before yet he dreamed he lived, but it's possible that the Heart was the only thing allowing him to create such an avatar. With the Heart gone he lost his anchor here on Mundus, whether or not he could regain it, whether or not he could dream again is up in the air. I don't think the Heart was the key to his immortality, but I think it probably had alot to do with how he managed to be more than a thought. As for after his death, maybe he now resides in the place of eternal witness, knowing the dream is his but unable to exert influence. Maybe he's devising some other way to regain himself from the mists of the dreaming-sleeve. Maybe he's recycled at last. Maybe he's completely lost within the madness of the godhead forever. I don't claim to know.
The whole idea about Dagoth Ur invading Oblivion instead of Oblivion invading Tamriel is interesting - can Daedra catch diseases like Corprus?
They may not be capable of catching the disease but it can also be used as a curse, reference Dagoth Gares cursing you with it when you kill him. The question is whether they can be connected to the Heart of Mundus, being from Oblivion. The Daedra are part of their Prince, therefore part of Oblivion, so its tricky.
Can a part of Oblivion become part of Mundus? Can a Daedra have its essence become that of Mundus? Can a daedric spirit be unified with the aedric? Well, can mortals and Daedra breed? 'Notes on Racial Phylogeny' says there have been no documented cases, but 'Varieties of Faith' also says that Molag Bal is known for corrupting mortal bloodlines - how could he do this if they can't breed?
That said, I think there's a decent case to be made that they could be inflicted with Corprus, or at least that Ur could modify it so they could be. The result is up for grabs. I don't think they'd become Corprus beasts, more likely they'd have some sort of mythic sensory overload and explode.
Would it be possible for Dagoth Ur to invade Oblivion had he lived?
I think so. "Dagoth Ur's Plans" says that
"the only opposing forces Dagoth Ur worries about are the Tribunal, the Daedra, the Emperor, and the Incarnate." At that point the Tribunal, Emporer and Incarnate would be dead, so it seems only natural that he'd turn his attention to the one threat to him that remains. It does also say that
"the Daedra represent no coherent obstacle to Dagoth Ur." Its true that they're not an obstacle to any of his immediate plans, but in the long run they may prove to be a threat, which is something I'd think he would take into consideration.
As Ur says,
"[Akulakhan] will serve as the prominent banner and symbol of our cause -- to defy the Empire, to liberate mortals from ancient superstitions, and to glorify our crusade against the gods." This might not be explicitly referring to the Daedra, but I imagine they're implied within the mix.
Of course, once he conquered Mundus he may have no need to invade Oblivion. The sheer power of Akulakhan as a tower may make the barrier of Oblivion completely impenetrable for the Daedra, making it so Ur never had to worry about them anyway. Furthermore the sheer mythic consequence of him ruling Mundus may make the Daedra so weak that they couldn't act against him if they wanted to - personally, I don't think we know enough about the Daedra to say one way or the other.
Now, in terms of whether he
could, again, the mythic makes it sketchy. He'd certainly have the raw power to. His armies are stronger, respawn more quickly and have a weapon which would make a Siege Crawler (or even the Numidium) look like a child's toy. But the mythic makes it sketchy. Could Dagoth Ur or Akulakhan even enter Oblivion without the universe crumbling under the weight of paradox? Again, I couldn't say...
I never heard that, is that in this thread or somewhere else? And regardless, neither we nor Dagoth Ur's followers hear from him again following being his being severed from the heart and killed.
Well of course you don't silly, no archenemy worth his goatee is gonna come back right after he's been defeated - gotta have time to scheme, devise plans, and find a fluffy white cat to stroke (and you know that last one is gonna take a while in Tamriel).