Belarus could be made to easily rejoin this Communist Russian State. I mean they are still a command economy and want to join with Russia now.
Ukraine probably not so much. Strong independence movement. Moldova would but its separated by Ukraine which would make it difficult. Might be a war to reunite it with Romania in that case.
Expect conflicts in the Caucus and a lot of internal upheaval. Economic disaster.
Massive Emigration.
Ukraine is a highly divided nation, still them joining a new union having left the old one is unlikely. Even so it is possible as is some sort of loose confederation, like the CIS only workable.
Oddly enougth Kazhakstan might join a union with Russia & Belarus too, if Russia seemed to be recovering...