hmm personal union between the byz and the ottomans seems like a bit of a stretch. most plausible scenario is no centralized dominant power in anatolia & the balkans but bickering statelets.
but if somehow mass conversion of turks to orthodoxy (including the little beys of Karaman Candar et al) and the personal union happens, the ramifications for future ottoman lands & vassals are quite interesting. I can see orthodox tatars in crimea, orthodox 'crusades'to liberate aleppo lebanon and the holy and, cultural & religious integration of the balkans (which butterflies balkanization).
Also, orthodoxy itself would be altered, with the turks' expansionism instigating the said 'crusades', the relationship between the church & state (ottomans were big on centralization and dominant state), cultural intermingling a la andulus mudejar architecture, interesting relationships between the byz church & the armenians, copts, syrians (heretics or cousins in religion?).
Oh, we can see two russias with the muscovites in the north and the Kievan Rus & orthodox tatars in the south. As for the stability of the turco-byz emire, i'm sure the ottoman dynasty (Osmanoi??) will waste no time in eradicating influential byz families (doukas etc) and bring a centralized state instead of the themata.
If an orthodox schism can be avoided, this orthodox state can persevere in the face of future nationalism (thanks to religious & ethnic unity). Venice is screwed big time (dalmatia, crete & and all your greek islands are belong to us) & no Genoa in the black sea. not sure about the status of jews & catholics within the empire. Turkish identity could be assimilated. The Crescent and star would remain the symbol of constantinople. The empire would keep its claims of being the new rome (the turks already called it Rum) against HRE and the Russians. bonus points for orthodox (southern) hungary & sicily.
And who knows how many possible philosophers, artists and inventors were killed/exiled in the fall of constinople? No steampunk but at least renaissance can arrive in eastern med as well.