I think the goal should be to tell them we have no interests in Syria and that Russia can keep their base and influence there. What we want is what the Russians want, that is stability in Syria and not having it as a breeding ground for radical Islamicists. The majority of these warring factions would put down their guns and end the civil war if there were someone in power other than Assad.
Our goal should be to have Russia usher Assad into a nice quiet retirement on the Black Sea somewhere and put a hand picked successor in. Someone like Al Sissi who came to power in Egypt. A strongman type who is going to keep a lid on the crazies and not be out to bomb and gas his own people. Someone the majority of Syria could unite behind and then we can work on wiping out Al Nusra and ISIS. It would be our job to ensure that the Kurds in northern Syria unite behind the new leader. I’m sure that Russia knows who among the Generals etc.. could be that successor. They know all the players.
But what about democracy and civil rights? [And the question is sarcastic in case anyone missed it - this does not appear to be an option in the ME].