You'll probably need a strong balkanic state. The way I see it, Byzantines were out of course in post-1204 era, Serbian Empire was a mock Roman Empire, letting Bulgaria on the run.
Nerfing Bulgarians wouldn't be that easy, but can be done : more crushing victory against Latins (enough to take Constantinople, but not Greece proper) followed by an alliance with west (OTL, Tsars were at first quite close of Latin church, allowing for a lasting union), preventing Mongols raids in Hungary and Bulgaria...
An imperial Bulgaria could be more cohesive, and avoid its OTL desintegration at least enough to prevent Ottomans to get an hold on Balkans. It may, but it would be quite of a wank, sattelize not only Latin states but anatolian Greek states (admitting first that Mongols blew the shit off Turks, letting Ottomans in existance but in a reduced situation).