Or for an even further POD: Austria retains Wallachia and gains Moldavia upon defeating the Ottomans in the late 17th century.
I had a similar idea, with Austria somehow gaining Wallachia and Moldavia during one of the Ottoman-Hapsburg Wars. Perhaps in TTL the reigning Hapsburg monarch is crowned King of Romania (perhaps called Dacia in TTL?) as well making for an interesting situation in the Balkans when/if Russia begins to ascend...
Perhaps Austria develops some ports in the Black Sea?
And they can control the mouth of the Danube if they also take Dobrogea.
If the Hapsburgs control Wallachia, Moldavia and bits of the coast, they cut off Russia from a land bridge to the straits and to the Balkans as a whole. That should increase Hapsburg-Russian tensions considerably. I'd rather go with Hapsburg Wallachia and Moldavia, yet Dobrudja, the mouth of the Danube and the coastal part of Bessarabia go with Russia.
In any case, this requires Austria being more successful against the Ottomans and likely for a longer time period. Continued cooperation with Russia can do this. Yet if Austria takes Wallachia and Moldavia due to their successes against the Ottomans, I think you may take as granted that the whole of Serbia is Austrian as well ITTL.
They did actually come close a few times: in the same period as the brief Austrian Serbia ST mentioned they held western Wallachia ('Oltenia'), and then during the Crimean War they occupied both principalities and a lot of people in Vienna saw no particular reason to leave, although of course in the end they were neutralised. And even after that, one of Napoleon III's whackier schemes was that Parma and Modena should be annexed to Piedmont as part of a federation of Italy and the Austrians would be compensated by seeing their monarchs take the thrones of the principalities under Austrian protection.
One thing's for sure: with the best part of Romania under Hapsburg rule and ideas freely being exchanged over the Carpathians, Transylvania will be a lot more restive. Perhaps the Austrians help the Romanians as a counterweight to the Hungarians, like in Galicia; perhaps the Romanians are just generally discontented (I wonder whether the Austrians would try and impose Greek Catholicism? Probably not by this point: they never minded the Serbs). Either way, they kind of systematic Magyarisation of Transylvania you saw IOTL will be difficult.
Transylvania is still more mixed, what with the Szekelys and Transylvanian Saxons predating the Great Turkish War, but of course it won't be Magyarized as much as OTL.
The Hungarians won't like any scheme of Vienna to divorce it from the Kingdom of Hungary regardless.
They did actually come close a few times: in the same period as the brief Austrian Serbia ST mentioned they held western Wallachia ('Oltenia'),
One thing's for sure: with the best part of Romania under Hapsburg rule and ideas freely being exchanged over the Carpathians, Transylvania will be a lot more restive. Perhaps the Austrians help the Romanians as a counterweight to the Hungarians, like in Galicia; perhaps the Romanians are just generally discontented (I wonder whether the Austrians would try and impose Greek Catholicism? Probably not by this point: they never minded the Serbs). Either way, they kind of systematic Magyarisation of Transylvania you saw IOTL will be difficult.
The best POD is a Austrian victory in 1737-1739 against the Ottomans. They had already annexed west Wallachia in 1718. If they won, they would likely take the rest, and with Moldavia cut of from the Ottoman Empire it would likely fall too.
The development would be interesting, but depend to high degree on the precise extent of the Habsburg gains. If they gain coastal Romania and the Budjak region, which at this point had low population and it was mostly Muslim. The Habsburgs would offer them to convert or leave, and settle it afterward with Catholic settlers. So we would get a coastal area populated by Germans.
The Habsburg would likely develop the Danube to increase trade, and we would likely see a port being founded in the delta, through it will likely have little economic relevance until the late 18th early 19th century.