Serbia is an independent country currently negotiating an EU-memebership. In order to get to Serbia, Russia would have to go through at least one EU-contry or Turkey.
1) its economy is far too screwed up for the EU to include
2) there's still the Kosovo problem, that won't go away soon
3) Croatia and Slovenia will veto an EU membership of Serbia.
It is more than likely that Serbia will turn to Putin instead - and when Putin says "shut them down", Serbia will comply.
Could say the same about Ukraine
Transistria is at border of Ukraine and Moldova, owned by both, with roughly equal populations, but both Moldovans and large part of Ukrainians were at same side. For example, leader of separatists was captured by Moldovan police in Ukraine capital city. Why "conflict" happen if both native nations were at same side?