You don't need to complete the stormcloak rebellion for that. Officially Ulfric wants a "free" Skyrim (Nords only) because he's pissed off by the empire's politics and the weak emperor that lets the Thalmor run across the country and torture random people for having the wrong religion. Inofficially, Ulfric simply wants to rule Skyrim (Nords only) . Period. He has no ambitions of attacking Cyrodiil and High Rock (aka the empire).
The empire wants to consolidate its remaining power, so that humans have a future, it will be hard enough to win back Hammerfell, and restore a land-connection between High Rock and Cyrodiil. Loosing Skyrim will destroy the empire, it's divide and rule for the Thalmor.
So the only possible response to a victory of the Stormcloaks is not to accept the secession, and blockading the pass to Riften, and the sea routes to Solitude / dawnstar. Hidden imperial camps make sense, what doesn't make sense is that stormcloak soldiers and imperial elite troops stand peacefully next to each other in the streets of Solitude.
The only problem with your idea is that Hammerfell doesn't
want to get back in the Empire. They're supremely pissed off that the Empire kicked them out of the fraternity for simply trying to protect their land. So when the Empire booted them, they fought back and
single-handedly beat the Thalmor and drove them from all parts of Hammerfell!!!
Obviously there is a ton of bad blood between Hammerfell and the Empire right now, but Hammerfell beat back an entrenched Thalmor enemy, a unified Sovereign Skyrim can easily do the same. The Empire
needs Skyrim, but Skyrim
does not need the Empire.
So I do agree with you that High Rock will almost assuredly seceed from the Empire now that there is no connection between High Rock and the Empire. Also, as the Dunmer rebuild it, I can easily see Morrowind becoming its own Sovereign Nation again as well. They were doing fine before the Empire, and even once they signed the treaty with the Empire, Morrowind was the only
Autonomous Province in the Empire, meaning that the Empires lackeys that were stationed inside Morrowind's borders were there for merely a show, but having no real power. Morrowind will be just fine after the Empire is gone as well, in fact I personally think that they will come back stronger than ever as a Sovereign morrowind.
Near future, here's what I think goes down:
Morrowind officially seceeds from the Empire (already pretty much has happened, but they make it official). Resolutely resists any attempt to absorb their new nation into any existing or future conglomeration of nations. Gives the Dominion and the Empire both the finger, signs mutual Peace & non-agression treaty with Skyrim.
Skyrim seceeds and is its own Sovereign Nation again.
Hammerfell rejects Impirical pleas to rejoin the vastly weakened Empire, stays as its own Sovereign Nation, but works out a peace treaty with Skyrim where they have mutual military protection clauses written in for both countries sakes.
High Rock seceeds and either aligns with Hammerfell, or bargains for safety with the Thalmor. Possibly becomes an autonomous province of the Aldmeri Dominion.
Orsinium will not be a factor as it was sacked and is now Breton property. Orcs have no homeland and are split amongst these five factions as simple, paid mercenaries.
This is how I can see it working out based upon the Lore given so far in the Fourth Era.
