Has Bethesda ever addressed this?

They haven't as far as I know, but the usual response to this question is that it won't matter in the lore after the game and that they'll come up with an answer to explain it. Something along those lines anyway.. look at Daggerfall and its numerous endings for example.

I personally wanna say Stormcloaks solely for the reason that it would make the future more interesting with an independent skyrim.

They could work it out so that it doesn't matter which side you pick.

For example, regardless if you support the Empire or the Stormcloaks, the Thalmor are defeated, the Empire crumbles and a new one is formed, Ulfric and Tullius both die and are honored as heroes, Skyrim becomes independant, etc.

I want dialog selection in the next game on who won. if were in a different Province the stormcloaks and tullius Legion wouldnt really have a prominent role in the next game It really wouldn't be that hard to have different winners maybe a mentioning or two (conversations,books) possibly a special mission based on who won . I don't want a canon ending or "it's a mystery..!!!" ...

Their usual approach is that something with bigger repercusions will be waht is remembered; maybe they will say how Skyrim was torn by civil war and focus on the return of the Dragonborn and whatever happens on the expansions, with no mention of the result of the civil war at all

Ulfric and Emperor will both die under mysterious circumstances, the Empire will morph into something new, and the Thalmor will collapse from within.

Skyrim was torn apart by dragons/civil war/Volkihar uprising thus no one knows who had the upper hand in the war. Some say that the legendary dragonborn of the 4th era pick a side, but surely hero of this magnitude didn't trouble himself with little rebellions.In the aftermath of Alduin's return both sides took colosal loses. General Tullius and Ulfric Stormcloak died on the battlefield and was declared High King/Queen of Skyrim by the moot to calm the infighting. While Skyrim was getting it's independence the empire collapsed after the assasination of the emperor. The rising threat of the Thalmor gave the lost provinces a common enemy, thus the Second Great War was victorious by a coallition of former imperial provinces. Ending the Thalmor threat to the land. Commoners say many outstanding things about the dragonborn who is the real legend of those days. Some say that he was a vampire or a werewolf, some say that he was both, others say that he could turn into a dragon also some say that was the behind the assasination of Titus Mede II, but no one believes such lavish stories.

It won't be set canonically. If I were a betting man, I'd say Elder Scrolls VI will be on Summerset Isles and one of the factions will be the Alliance, which has come to put an end to the Thalmor. Admittedly, it could be ANY province disputed and the ambiguous Alliance of Hammerfall, Skyrm, Cyrodill, Morrowind, and High Rock has come to drive the Thalmor back. The Alliance would still be The Alliance, regardless of who won the civil war, as Hammerfall is still independent, even if you do the Imperial quest line.

It will probably be kept just vague enough so that both supporters are satisfied.

Sheogorath will decide. Who would dare understand his thought process?

It will probably be something along the lines of 'neither side one, because the war with the Thalmor resumed and united the two sides again!'. Something like Daggerfall and the Dragon Break ...
If there had to be a canonical ending, it'd probably be the Stormcloak victory.

Most likely neither.