I think your observation on the potential for a semi-autonomous — or autonomous — rainbow state — Kurds, Sunnis, and returning refugees discontented with Assad — is a very real possibility. And we might see the US (and the ever-faithful coalition) creating a protectorate (Palestine after WW1).
The pre-conditions are all there. US involvement cannot be simply a matter of IS-cleansing and bye-bye — you’re very welcome Mr. Assad, we don’t mind cleaning up your country for you. The US wants some level of influence over the territory East of the Euphrates. It does not want Iran to have free access to Assad’s Syria. It can then say to Israel “We’ve weakened the link between Iran and Syria.”
It is by all accounts a remarkable development over the last 6 yrs. — from support of insurgents (Clinton’s idea) to active personnel on the ground w. material. IS is a convenient imposition, allowing deeper investment and militarization in the name of combating the most hated group of people on earth. Investments like this always mean in the end a return in equity.
Russia, which understands Realpolitik, may find an East of Euphrates land deal a liveable solution. Assad stays, the Kurds and insurgent factions are appeased. Even Turkey may feel amenable to a Kurdish homeland. Assad may have no choice if Russia agrees.