Moscow could extend power to control Belarus/Poland, Finland, Baltic and Ukraine.
Also of note, lithuania/Belarus, Poland, Finland, and Ukraine were not always keen to be ruled from moskva as well., these states just didn't get a choice in the matter.
Ukraine and Belarus was considered core territory in the minds of Moscow and St Petersburg
Balkans are just to far out of reach and honestly while they are slavic/greek/turk with other groups thrown in. Russian they are not, nor did they ever want to be.
So, You would need 1. No Russo Japanese war, no world War 1 and more than likely no Nicholas II. You also have the Austrians in central eastern and Southern Eastern Europe, the ottomans and Italians.
Serbs and Bulgarians wanted to rid themselves of the ottoman yoke not gain a new master, same with Greeks.
Poles wanted independence and not be russified.
Start with a Russia that wasn't trying to russify other Slavic groups, but liberal enough to form a large loose union of independent nations in some form of giant trade/economic block