Well, I'd say the best bet would be to have it comprising only Russia, Belarus, Kazakhstan and PART of the Ukraine (IE having an independent Ukraine), that way you get rid of the major nationalistic problems.
Those nationalist problems have been rather exagerrated by the historiography of the victorious. If the more fundamental problems that led to the failure of the effort to reform here referred to, then retention of all Ukraine and Central Asia without trouble is perfectly plausible. The baltics are an interesting question.
So, what were the problems that faced Gorbachjov's efforts? Two biggies: he was trying to drastically a system that was so stagnant that tweaking it caused it to disintegrate, and his plans were outrun and undercut by the new politics unleashed by Glasnost (Yeltsin played a major role in the dissolution, strange as it might seem).
The solution, it seems to me, is an earlier, more moderate change which allows the USSR a more flexible and competative economy, but with *Glasnost not happening or not happening until later. Echoes of China, here. Not having WW2 would help a lot. Besides seeing off teh genocidal horrors that befell the Soviet peoples, it gives you the possibility of excluding the Baltics and the most nationalistic bits of the Ukraine, and IIRC puts the Russians in a better demographic position in various places. Not only this, but you probably don't get a "Cold War" dynamic in the same sense, so no having to hold down eastern European vassal states, or rather, failing to, and less money being sunk into the nuclear race or Afghanistan.
So, no WW2. Post-Stalin leadership succesfully implements something Perestoika-ish much earlier on, while maintaining totalitarianism and the power of the Soviet state. Done and dusted, pretty much. EdT does something like this in "A Greater Britain" with Bulganin and Kosygin.