A Transylvania that doesnt go with the Hungarian uprising will most likely have the Romanian population in a dominant position, though, or not? So there would be few reason not to unite with the other two Romanian grand-principalities, or at least with Wallachia if Moldavia becomes Russian.
Well, you'd think so, but "Transylvania" in the sense of its people didn't go with the Magyar revolt, and of course the revolt failed. In the next decade, Austrian policy was basically Vienna Rulez Okay. Everywhere was placed under tightly centralised neo-absolutist governance, but this meant that while for the aristocratic Hungarian and Polish revolutionary types things got worse, for the peoples who had been most voiceless before 1848 they got better. The Austrian military regime in Transylvania gave Romanians citizenship and abolished serfdom, while going back to the Josefian policy that if you wanted to get anywhere, you'd better learn German.
The natural development after neo-absolutism ended in 1859-61 would have been for Germanisation to be ditched, more power entrusted to local assemblies, and the Austro-Romanianism to prevail, as happened in Galicia; and indeed, for a few years that was the plan.
1867 was a total disaster for the new pseudo-federal ideas, which would hopefully have made the Hapsburg monarchy both liberal and effective: the most advanced and hegemonic national movement, the Hungarians, took the opportunity to seize the state hostage and crush the Serbian and Romanian institutions (and embryonic Slovak ones) cultivated by Vienna and even subordinate the Croats.
(The Poles, of course, had an equally advanced movement, but between the end of military rule in 1865 and the new status-quo of 1873, Galicia had effectively become another component of the monarchy in the breadth and dept of its autonomy: it never made the status formal simply because that would have ignited yet another constitutional crisis when the Czechs came along for their slice of the pie).
If we were to imagine Austria winning in 1866, it seems that the development of autonomous Transylvania would have continued within a "devolved" Hapsburg empire. But since that province would by its very nature come to be dominated by the Romanians, it seems to me that it would either be part of Austria of part of Romania.
(An interesting footnote to the whole thing is that when they occupied them during the Crimean War, the Austrians thought about keeping Moldavia and Wallachia if it proved at all possible, and Napoleon tried to get them to swap the Danube states for Venetia just before the 1866 debacle. What about a united Austro-Romania?)
1848 could be a turning point here; Transylvania was til then a Grand Principality with a viceroy under the Habsburg crown. The leaders threw in their lot with the Hungarians more or less by a pro-Hungarian coup (which was one of the reasons the Russians were involved even before they invaded Hungary proper).
Did they? I was only aware that they assisted the evacuation of the brief loyalist-Romanian coalition after Transylvania was invaded by the revolutionaries to bring it to heel, and only invaded it in earnest at the same time as the rest of Hungary.