It's quite possible the Latin Empire manages to extend its control along the Via Militaris up to Serdica, forcing the Bulgarians on the more montainous parts of their country for a while, and generally does better against Nicaea and Epirus with far-reaching consequences.
That’s pretty much how the second Bulgarian Empire started off initially, at least until the Byzantines bungled their military response allowing for the Asens to move on the offensive and dominate the Balkans.
Though assuming events go similarly with the Laskarids, would they perhaps flee North and take over Bulgaria instead?
The reason why the Byzantines couldn’t muster an effective response was because of the misrule and cowardice of Alexios III. His policies effectively led to the Empire more or less becoming decentralized. The people began looking towards local leaders to solve their issues.
Without focusing on Bulgaria for the 1180s and 90s, the Roman’s are mainly going to be focused on Serbia- before the crusaders arrive and shake everything up from the centre, I think Serbia would also return to being a Byzantine vassal.
That depends on how Bulgaria is. If it’s after the Asen revolt is suppressed the Empire has bigger things on its plate to worry about. If there is no revolt, I could definitely see this taking place.
all Stefan Nemanjic had married Eudokia Angelina, giving him a dynastic claim similar to the Laskarids of Nicaea, and given stronger Byzantine power preceding the fourth crusade, he wouldn’t have abandoned her in trying to get the pope to give him a crown.
Honestly the Latins could possibly play off the two sides.
If the Laskarids take the Bulgaria I could see the Komnenians of Trebizond easily take over Anatolia. Of course there was Philadelphia which also had an Imperial claimant, but considering how he was beaten by the Nicaeans in otl, the Komnenoi could do the same thing.
I think a Bulgaria that’s still under Roman control is going to greatly change things as that’s one less major enemy for the Romans to deal with as a threat and one more major foe for the Latins as whoever ends up ruling Bulgaria will have imperial ambitions for Constantinople and the empire as a whole. Whoever controls Bulgaria will be a major contender for who will reunite the empire. Especially since they’ll likely be in a good spot to rapidly retake the European side of the empire. If they’re linked up to the nicean empire even better. Overall I think you’ll see a ERE that is restore earlier than IOTL.
Honestly the Latin Empire in otl had no business being as successful as it was considering how dysfunctional it was. Still with more resources I could see them take potentially more of Anatolia or something even making the Duchy of Philippopolis into a more viable polity. Then again the city did not welcome their Latin rulers well. They went into open revolt and the guy who was supposed to rule it, left fearing his safety.