[simplification] Uniates are Orthodox Christians who accept the authority of the Pope. [/simplification]
There are clusters of them all over Eastern Europe and the Middle East: see
http://en2.wikipedia.org/wiki/Uniate_Church, which lists 22 distinct Uniate churches, though some of them are quite small.
However, there are no Uniate Serbs. What if there were?
Well... probably the best way is to have Austria-Hungary not lose that border war with the Ottomans in the 1730s. Austria had picked up some nice frontier territory in the 17-teens under Prince Eugene, including Belgrade and its immediate surroundings. OTL they foolishly frittered it away a generation later. That had a number of unfortunate results, the main one being that they didn't manage any more territorial expansion in the Balkans until they picked up Bosnia 140 years (!) later.
Say that in this TL, though, the Austrians manage to hang on to Belgrade. Now they have all the best bits of what's going to be Serbia, including the Metropolitan seat of the Church. Serb nationalism in the Ottoman territories is going to be hamstrung, since an independent Serbia without the pashalik of Belgrade doesn't make much sense.
Now, just to make things more interesting, have the Austrians force the Serbs of Belgrade and Vojvodina to become Uniates. There's precedent, in Transylvania, Ruthenia and the western Ukraine. OTL, Austria forced Uniate status upon Orthodox churches there. It met with resistance at first, but eventually came to be accepted as the normal order of things. Today there are hundreds of thousands of Uniates in Ukraine and Romania, and they vigorously resent and resist the idea of being pulled /away/ from Rome. (And -- in Romania, at least -- get beaten up by nationalists accordingly; Ceausescu hated the Uniates, and the contemporary Romanian Orthodox Church has picked up where he left off.)
Anyway. This adds a fourth group to the Serbo-Croat religious mix -- there will still be plenty of Orthodox Serbs in the Turkish lands. Paradoxiclaly, I think this will make it easier for ethnic/linguistic nationalism to take root. Uniate Serbs could serve as a bridge, after all; they'd be just as "Austrian" (and so "civilized") as the Croats, but Orthodox rather than Catholic. If Serb nationalism is definied as "all Christian South Slavs" instead of "all Orthodox South Slavs", then the Balkans are going to look very different.
Of course, Austria keeping Belgrade would yank Balkan history out of its track pretty quickly anyway. There's probably no Serb Revolt and so no Kingdom of Serbia. I think it's plausible that Bosnia and much of the rest of Serbia could come under Austrian domination before the French Revolution. In that case, the *Yugoslavs would be in roughly the position of the OTL Czechs and Slovaks: most of them live under Austrian rule, and nationalism can coalesce along one pole (cultural expression -> autonomy -> independence) instead of two.
As an interesting side effect, you've also got a richer and (probably) stabler Austria.
Thoughts?
Doug M.