Hard to imagine any powers being able to take Constantinople by storm in the style of Mehmed during the early/mid 15th century, other than the OTL Ottomans. This does not mean that the ERE isn't still doomed, or even that it will survive another generation, but the Ottomans were a uniquely capable force in both quality and quantity during the 15th century in the Balkans. Most likely outcome of a total destruction of the Ottomans at this point is outside powers becoming the leading forces in the Balkans, Hungary plausibly enough but also the various Italian players, while a wide range of local entities will have some breathing space. Bulgaria could be re-established plausibly enough, and the ERE under someone reasonably competent (i.e. Manuel II as OTL) could plausibly regain parts of Thrace. More than anything though this will be a region in flux, impossible to predict the most likely outcome of the century.