Two, complementary, situations could help.
Let's assume Huns either never form a cohesive migrating group or simply fail at establishing their domination in eastern Europe, meaning Sarmatians, Alans and eastern Germanic peoples (themselves more Sarmatised than IOTL) would dominate the area without the geopolitical changes than Huns provoked.
I'd see a good part of Proto-Slavs being ingrained into stabler entities between Black Sea and Don basin, that (if things goes more smoothly for the Roman Empire) would be similar to the upper Danube and Pannonia IOTL, being the center of tribal hegemonies (probably Alan or Sarmatians, at least heavily under their cultural influence) from Chernyakhov culture basis.
A stronger North/South attraction rather than West/East could be enough, not to prevent Proto-Slavs to expand eastwards but to limit it (as for preventing Slavs to be present in Ukraine and Belarus, that seems hard giving it's most probably one of their initial centers).
Preventing a Rus'-like entity to appear isn't that hard, once you butterfly away or at least delay long enough the establishment of Baltic/Black Sea trade roads trough Volga and/or Dniepr long enough.
It was the Arabo-Islamic trade roads that first motivated the establishment of strong entities, and alternate trade roads to Baltic in the case of a surviving Roman Empire (or even a maintained late Roman continuum dominated by ERE economy without the VIIth crisis).
Now, structuration of the region can't be delayed indefinitely, but you could end with Sarmatized/Alanized south-eastern Slavs, and northern Proto-Slavs advancing less deep in East, (maybe forming a Rus'-like entity in Poland with whoever dominate Baltic trade).
Eventually Finno-Ugrian would have a better deal, though I suspect they might end as an equivalent of Balts when it come to rivality with their neighbours.