What if during the peace negotiations concerning the Crimean War, Austria offers to purchase the all of Bessarabia up to it's black sea coast. France & Russia would be very much against this, but if Austria manages to convince the Ottoman (it was nominally returned to them via Moldavia I believe) money to cover Ottoman debts and promises of neutrality in the event of another war with Russia, if Russia can't pass through Bessarabia only true land front is the Caucasus region more defensible & possibly relatively easier troop concentration. GBR might be convinced with this as well as it'll contain Russian influence and her threat to the Ottomans (more specifically the straits of constantinople).
Assuming this goes through, and we still have a Russo-Turkish war of 1877, seeing as Austria already has control over most of the regions inhabited by Romanians they could unify with them under the Habsburgs with tactic agreements of autonomy of course. Too much of a stretch?
Habsburg benefit: port on the black sea at the end of the Danube, assuming more naval investment & increased trade with the ottomans doesn't seem like a bad deal, plus with that in pocket she maybe less inclined to getting involved with Bosnia (maybe just take the Croatian populated parts). Port on the black sea would give her another outlet as opposed to the potential bottleneck on the adriatic.