Wait, what? You didn't read the Thalmor dossier about Ulfric, did you? It clearly states that a Stormcloak victory is to be avoided at all costs.
The OP was very insistant he joins a side, players like to play all the content no matter what. In which case the Stormcloak victory is the lesser evil as noted by the dossier - but best case scenario, as I said and the dossier clarifies is that the war be kept boiling over as long as possible to maximise damage to both sides and to pave the way for Thalmor occupation/liberation/annexation.
Huh? The one side that's crying out for thalmor blood is more likely to form an alliance with the thalmor than the ones currently obeying their every whim? What?
Skyrim alone is weaker than skyrim under the empire. They will fold to the Thalmor, the empire has machinations of their return, which is why they want skyrim so badly - but yeah I spoke out of line there thinking about it again. Thalmor would never allow them to worship Talos which is the one thing Ulfric cannot give up since he's made it his rallying call (Even if it's not his actual belief, his actual motivation just seems to more about power and vindicating the sacrifices in the great war), so an alliance would not be possible as Talos worship is the one thing the Thalmor can't let stand as it's counter to their own objectives.
A seperate skyrim is still easier to conquer
If he's taking a khajiiti approach to things, the Khajiit worship the moons in the sky as well as Azura. The moons in the sky are Lorkhan's rotting flesh. Talos is mantling lorkhan which means the 2 have become indistinguishable.(In the same way the CoC is now Sheogorath)
I asked the lore guys about this, seen it quoted before and they couldn't agree whether Talos IS Lorkhan's mantled successor or not. But I
have pondered whether there'd be a long term problem. The khajiit might not worship Talos, but they are irrevocably tied to lorkhan, and it's Lorkhan more than anything that robbed the elves of their immortality. As for how the khajiit take it if or when it was revealled they were tricked with the moons; you only have to look at how the khajiit have benefitted from Thalmor rule in contrast to the Empire - I'm pretty sure I'd forgive them ;p (The Dutch conquered England which ended the dark ages and paving the way for new development without the yoke of the church; there is such a thing as the right invasion at the right time).
incidentlly, the player character seems to be walking the same path as Talos/Tiber Septim, are they implying that the Dovahkiin is going to take up Talos' mantle? - Given Talos led the nords to conquer tamriel where does this leave the khajiit or argonians... Argonian or khajiit conquering Tamriel would certainly mess with the timelines xD