Even Mournhold has been rebuild, as far as I know.

Nope. I have no idea how you can think this after you played Dragonborn.
The DLC disproved the theory.

yeaa we already been over that...

I am a PC owner, so I have not had the chance to play Dragonborn yet. Did something change in the DLC where Redoran is rebuilding things in Morrowind and people are returning there? All I see in vanilla Skyrim is that the Dunmer fled from Morrowind to Skyrim and to Solstheim, which the Nords gave to them. I have never seen anything in the vanilla game about Dunmer actually leaving and going back to Morrowind.

A couple Dunmer at least in vanilla wish to return to Mournhold. Malborn is fleeing to Morrowind. And yeah.. Balmora is rebuilt from what we know. Telvanni still live on Vvardenfell. The Argonians didn't abandon Morrowind, they were pushed out by Redoran.

It is stated by the second councilor of Solstheim, who goes on to say that the Empire abandonned Morrowind during the Oblivion Crisis. A detail I find curious, because Ocato in Oblivion states that no generals have agreed to send Legions back home.

The Argonians aren't really capable of conquering and occupying countries, the Hist just point them in a direction and tell them to kill everyone that-a-way.

Given what happened to the Hlaalu, I think it is safe to assume there is little to no Imperial influence in Morrowind anymore.

this^^ was my point

There's a conversation in the Ragged Flagon between Delvin and Tonilla where Delvin mentions how argonian patrols make it difficult to smuggle things out of Morrowind, which implies Black Marsh still has some sort of presence in Morrowind.

Everything in Dragonborn seems to imply that the Redoran fought them back.. and they certainly wouldn't leave any alive should they be "patrolling" Morrowind.

Morrowind will definitely be part of the empire if the thalmor threatens them. Dont forget the altmer despised them so naturally the dark elves are going to be wiped out along with the men.

What speaks for Morrowind′s de facto independence:

+ Redoran building up a standing army in the most formal sense, at least partiall abandoning the feudalistic army-style of Morrowind′s Great Houses before the Red Year.
It has hard to believe the Empire would allow this, since the Empire wasn′t capable of conquering Morrowind when Morrowind didn′t have a standing army at all (the professional imperial Legions failed against feudalistic House Armies).
And why should Morrowind need a standing army in the first place if not to become less dependend on the Empire′s Legions, who are de jure responsible for Morrowinds "protection" against foes like the Argonians.

+ The Empire′s Legions withdraw from Morrowind during the Oblivion Crisis. Yet, there is no mentioning of them returning. Beeing part of an Empire without any sort military presence in Morrowind is possible, but unlikely, since the Empires rule over its provinces is not of the hegemonial, but of the occupational type.

+ We don′t see any Imperial Influence in Solstheim; however, it is only a small part of Morrowind.

+ Some vaque dialogue-lines.

--------------------------------------------

Speaking against Morrowind′s independence:

+ No one claims Morrowind is de facto "free" from the Empire expressis verbis.

The Thalmor would not directly assault Morrowind. The Thalmor would probably try to rile up the Argonians into attacking Morrowind again. The Thalmor were known to have helped prod the Argonians into raiding Morrowind during the Red Year. The Thalmor are probably too afraid of having their [censored] pushed in by Divayth Fyr.
The Dunmer would not ally with the Empire either, since they know the Empire would not protect them. The Empire would tax Morrowind as a province, but as soon as [censored] hit the fan in Morrowind, the Empire would pull out again.