Your challenge is to have a rump USSR emerge out of an incomplete collapse of the USSR along the lines of today's "Red Belt" region of Russia. The late 80s/ early 90s can be changed as much or as little as you want, but the POD should be no earlier than after Gorbachev taking power. This rump USSR would exist alongside Russia and the other new republics (of which there may be more due to the chaos I imagine would be required for a Soviet state to re-emerge from the ashes of the collapse, if of course this is not completely ASB to begin with). The rump USSR may have relatively friendly or hostile relations with its "new neighbors," but you can decide how the politics of this plays out. good luck. BTW, the collapse itself can be as bloody or as peaceful as you like too. Minimal foreign intervention and no "Union of Soviet 'Soverign' Republics." Again, good luck, thanks for any ideas, and sorry if you find it ASB.