But at the same time,the Byzantine forces in Morea seemed to be much capable militarily.They were able to take over the entirety of Achaea and actually forced Athens into vassalage.They were also able to resist Ottoman invasion at one point.
Morea is a bit of a distinct case : it's rather representative of the various dynastical principalties that I mentioned in this thread (that existed at times in Thessalonica or Thrace), being largly autonomous from the imperial power (it's somehow reminiscent of a suzerain/vassal or apanage relationship, altough I'm not sure if you had an actual Latin influence there).
The decentralized success of Morea makes me think that, with the almost guaranteed establishment of similar principalties in reconquered parts of North-Western Anatolia, you could see an effective structural recover of these regions with a possible regional build-up. Nothing wank-worthy, but still...
It's worth noting, tough, that Morean success is partially due to the absence of genoese or venetian meddling : the principalty of Achaia was part of the Angevine sphere of influence (as the county palatine of the Ionian archipelago) and everything that weakened it wasn't seen with much hostility by these powers, that the least one could say.
It's why I think that a Byzantine Empire or byzantines principalties have a regional geopolitical role to play, allowing them to take back some part of the Romania's hinterland, on which Italians weren't that interested if at all.
But yeah,you've convinced me that if the Byzantines remained bankrupt and without an army much like the 14th century,they are screwed if the Venetians do try to take them on.
The big chance of the Byzantine Empire at this point is that Venetians couldn't care less about crushing Byzzies. They were fine treating it as a big client network managing the hinterland as long as their trade and coastal influence was safe. Their sheer naval hegemony in the region prevented Byzantines to go crazy there and after some clashes in the XIVth, the lesson was dutifully remembered by the ruling elite (much less so by the population, tough).
Venice wouldn't undergo a costly campaign to take on Romania's hinterland, an operation without clear benefits when it came to their interests : you'd really need some crazy despot, prince or usurper to force Venetians reacting with full strength in the XVth.
Nonetheless,it's entirely possible for the empire to take advantage of Venetian distraction on the Italian mainland.
It was what I tried to point in the last posts : if Italian Wars folds as IOTL, you could see Byzantine Empire managing to gain some hold back in the region. I don't expect a full-fledged war with the Serenissima, but rather making themselves necessary skilled auxiliaries (trough recruitment of Greek or Slavic mercenaries/retinues, that did were used IOTL but in relatively short supply after the Turkish takeover, while Venice beneficied from the influx of Christian refugees in its overseas dominions). I don't think it would lead to an immediate military build-up, but it could be part of its reappearance.
For what matter the XVth, tough, the empire isn't going to be that much relevant militarily, a secondary regional power mostly built on Latin,,Slavic and Turkic mercenaries.
Another thing is that the empire will probably be much closely aligned with the militarily much weaker Genoa than Venice as they already did to avoid being a client of any power.
It's not like they have a lot of choice on the matter : while Venice wasn't bound to puppetize the empire (the whole operation would have been costly, and would have met with both Romania's popilation AND Venetian elite opposition), it wouldn't have seen with that much benevolence a definitive shift.
Then, we have to remember that Byzantine, in the 1350's, did went at war with an Italian city-state in order to break its hegemony over Bosporus.
And it was a war against Genoa, with Venetians in the role of benevolent allies. On this regard, I don't think you'd have a natural inclination from Constantinople to Genoa (I think such view mostly comes from Genoese participation into Constantinople's defense in 1453), but rather a diplomacy of balance
While Genoa would have been more touched by post-Ottoman turkic piracy in the region (mostly in Black Sea) it would still have been an important regional power, not as much of a reliable ally on the long term than Venice, and if St George bank is still ITTL the financial heartbeat of the Habsburg hegemony, Genoa could very possibly make a comeback in the XVIth (altough the early part of the century is still going to be somewhat problematic, possibly leading to some Byzantine meddling in Bosporus proper).