We could look at the example of the civil war in Bosnia. When the Dayton accords were being drawn up, whoever controlled whatever territory they were in pretty much kept it. This unduly rewarded Bosnian forces, which had conquered a fair bit, but no one saw another way out of this morass.
In Syria we're in a similar morass. If we could get Russia on board whereby we would freeze everything where it is for a while and leave the different communities running the areas they control with an opening for areas under siege to get supplies delivered to them by the Syrian regime, and with monitors making sure that those supplies go in, then perhaps we could get some breathing room to take it further.