Which victory will be considered as canon, stormcloak or empire?
Both victories appears to have some extreme consequences comparing each one

I'd say, unless Bethesda intends to see the world return to be enslaved by the high elves, (Altmer) And unless Skyrim discovers some extremely powerful weapon against the high elf empire. The only possible canon victory would be imperial.

If Stormcloak wins, they would be a single independant province against the whole force of the altmer empire.
If The Empire wins, they will still have the strength of number, should their desperate pact with the altmer empire be broken.

(Going for both victories, so i'll find out in due time if the stormcloak approach is even survivable.)

But the Thalmor had the empire on the ropes and after the war, Hammerfell managed to push them out (even though they were weakened). I think they'll either do a dragon break type event or have some significant event that occurs that nullifies both sides.

To be honest I doubt we will see a canon victory for either faction. When TES VI comes out we will get some generic history of "There were revolts that may/may not have been successful". That said if there is to be one I hope it's the Imperials lol.

Regardless, unless some mayjor event (or miracle) takes place, Skyrim will stand a better chance with the imperials. I would know as I had a lengthy discussion about this with a friend of mine where I took the stormcloak side.
To my knowledge, in absence of any miracles or major events, Skyrim would never stand a chance as an independant province against the High elves.

I think it could go either way though, the Redguards (war weary themselves) repelled the Thalmor after the war (who were also war weary) by themselves. And if lore is anything to go by, Nords are very good at killing elves.

I hope some of this is addressed in the expansion. We're left with many open questions and I hope the expansion won't be something out of context like Shivering isles. While I found it enjoyable, I was a bit disappointed that I didn't add to the main quest the same way as Tribunal did (investigating the fate of the other two men-gods)

I think the stormcloak victory is cannon, but after the events of Skyrim the Empire will rebuild with leaders much more powerful than the current milk-drinkers in charge. Possibly led by your character, the Dragonborn, a la Talos.

What do you think the chances are of Bethesda implementing save game transfers for future games? Otherwise, I suppose we'll get some vague sort of stalemate or middle ground in what happened during the civil war for Skyrim. Or another Dragon Break I suppose? Warp in the North

To quote then the great Chinese novel of The Romance of the Three Kingdoms: "The general trend of All Under Haven: that which has been divided for a long time must unite, which has been united for a long time must break up." Long story short I would expect upcoming TES games to see the end of the Empire (I suppose in the next installment this end would be imminent, and in the one after the next it would past event). Short story even shorter Stormcloak victory will be canon.

I think the canon ending will be a Nord dovahkiin helping the stormcloaks achieve independence of Skyrim from the Imperial, and war with the Aldmeri Dominion.

If you read the book about the great war, the combined efforts of the Imperials and their allies were able to utterly destroy the Thalmor forces in the Imperial city. It really boils down to fighting alone against a fanatical Elven Empire or joining forces with Allies to drive them back. A lot of the books suggest that the Thalmor are essentially using a divide and conquer scheme. The Kajiht are a perfect example of that, as well as the Redguards in Hammerfell.

That's been my assumption all along. Given the frequent comparisons to Tiber Septim, I expect that canon will be for the Empire to have had difficulties and briefly fragmented (without specifying which provinces broke away and which did not) before a new Dragonborn appeared to walk in Septim's footsteps and forge a new Empire, perhaps even to the point of ascending as the Tenth Divine (although that could cause problems, as I would expect them to leave the Dragonborn's race unspecified and "we are offended by the notion of a human ascending to divinity" seems to be a major element in rationalizing the current state of affairs).