There was a plan for Syria: stop Assad committing genocide with the aid of the UN. It might not have worked but it might well have stopped or slowed the advance of IS. Unfortunately the Russians vetoed it.
Now, however, Assad (whose regime is probably an even bigger butcher than ISIS but unlike them not so much a threat at a wider level) is effectively finish, albeit much later than most expected. There is no prospect of him regaining even half of the country back, whereas at some point it looked as though Syria might split in half
Not sure I think much of this possible deal. IS is not really a threat to Russia, only to Russia's ally. I can only see them "rescuing" him as a sort of "thank you" for his previous services ... Not sure that him leaving the scene will do much to stop IS either. He is a pretty effective weapon against them, given his affinity for mass, indiscriminate bombings.