I would divide Slavic union in 2 sub-unions and one member besides them.
1) Slovenia, Croatia, Bosnia, Serbia, Montenegro, Macedonia and Bulgaria.
2) Slovakia, Czech R. , Poland, Ukraine and Belarus
All areas where high domination of one state ( Russia) could happen would go on sub-union level and not on partial state level.
but…that is if we ( other Slavs) or Russians even want Slavic union. Why would Russians want such a thing? What would they gain if they are so in front of others? Slavic union would be good for smaller countries like our south Slavic ones + Czech R. and Slovakia.
I understand perfectly that this union was imagined primarily because of our ethnic bonds but always the economic factor was involved and was very important.