How about a general Balkan war, in the style of 1911 and 1912, with each side calling for outside support? That’s the worst-case scenario.
How's about restricting yourself to realistic worst-case scenarios?
The Russians found out the limitations of their power back in 1999, when their Colonels' coup-de-main was thwarted by a denial of overflight rights for the reinforcing units, and since then have found something much more compelling than Serbian interests to pursue vis-a-vis Europe: Petrodollars.
As to the Serbs themselves, if they chose to go into Kosovo and have another run-in with NATO, with what would they do it, and which of their borders, across all of which they could then expect NATO forces to cross in response, would they leave undefended in order to do it? You see, they've only got 5 Brigades with which to work with, and that's not enough to defend Serbia itself in the first place, much less mount offensive operations against the rest of Europe.
Simply put, not even the Serbian Radicals are stupid enough to go to war over Kosovo, and stupidity is about they only thing they've got going for themselves.
I fear this could go south fast. What if Russia comes in? Should US boys die for Albania? Will the Europeans fight the Serbs or watch and write notes of protest. Do they want Russia to cut off the gas in the middle of winter?
If Putin settles the issue with tanks it will put Russia back on the track of superpowerdom. The USA with elections and economic problems will let the UN handle this one. If we do, its our first step backwards.