wishful thinking. First of all, Russia really cant force Assad to step down even if they wanted to (there's no evidence they do). And even more important, Iran is against it and its their manpower that does most of hardlifing on the ground for the regime.
And If anyone thinks that rebels fighting now in Euprathshield op would sign of on such a deal would be greatly mistaken. Vast majority of these forces fight also in the city (Zinki, Jaish Tahrir, Jabel etc)..
Yes regime is on the offensive and they reversed many of rebels prevous gains, but its result of huge influx of iranian backed militias on that front and russian airstraikes, not some backroom deal. Flow of weapons from Turkey is still pretty much steady and going as before. Aleppo is too important to be traded of just like that.