But if the Danubian Principalities do not exist than Austria and Russia will be clashing over the Balkans much more openly and much sooner than in OTL.
This in itself is an interesting point. You figure that based on the proposed OP, an Austrian and Russian annexation of their respective Danubian principalities by 1845 or so would be likely?
That does not leave alot of time for Austrian-Russian clashing ahead of the time of Austria's OTL ultimatums in the mid 1850s. Of course it could be very consequential. With Austria taking more revisionist action in the 1830s or 1840s, and Russia doing so in response, we could well see the Russians *not* want to help Vienna suppress the Hungarians.
The problem is Austria was focused on Germany and Italy during this time. It's only after the Italian and German unifications that Vienna turned it's attentions southwards. Austria had no interest in focusing on three fronts.
I wonder if the unification of Italy was important to Austria deciding Bosnia was important for two reasons: a) First, After 1866 Austria could *not* have Italy as a dependent sphere and obviously also Germany so the Balkans were the only direction left. But also, b) With Italy united, and incorporating Venetia, and possessing a modern ironclad navy, there was a plausible claimant to Austrian Dalmatia and threat to it that had not existed before, and this made additional inland ground-lines of communication to Dalmatia seem more strategically desirable/relevant/important. In the pre-Italian unification period (1815-1860) Dalmatia's neighbors in the Adriatic were just Austria, the perceived as feeble Ottomans and the definitely feeble Papal States and Naples, so this sort of defensibility rationale for Bosnia was not seen as relevant.
I like the idea of an Austrian Wallachia and Russian Moldavia, but the point about Russia not wanting Austria on the black sea is valid. Perhaps an Austrian-dominated United Principalities (including Bessarabia) and a Russian Dobruja and Budjak?
That's a potential solution, if "fairness" is a principal concern along with buffering the Black Sea and Hungary. The problem with this is that I don't see Russia giving up Bessarabia, which it has owned since 1809. I suspect in the short-run, Russia would have Moldavia, Austria would have Wallachia, and Dobruja would remain attached to the Ottoman Empire.
Might we see efforts at "Serbian unification" of Bosnia, Montenegro, and Serbia?
The Muslim soldiers and administrators and landowners would be dead set against such unification. Bosnian Serb peasants may want it, but will not get into political authority without outside intervention or a surprisingly successful "jacquerie" peasant uprising.
