The Venetians, Eporite and Nicean Greeks?
Obviously, the Venetians are upset. However, I am not sure they were in a position to much about it. Perhaps they could be bought off with trading rights and some outlying islands such as Corfu and Crete. Long term, we might see Venetian money pulling the strings of a Bulgarian puppet-emperor.
The Greeks likewise will not be please, especially given Kaloyan's record (Siege of Varna comes to mind). Keeping the Greek population of Constantinople will be rough for the Bulgarians. Perhaps their best bet would be to trade Constantinople to the Nicean Greeks in exchange for recognition of Bulgarian claims on Macedonia and portions of Eprius.
The western reaction? Is another crusade launched?
IIRC, Innocent III was not happy about the sacking of Constantinople. The quick return of Constanitinople to eastern hands may be seen as divine justice for the sacking of a Christian city against the pope's orders. A crusade to retake Constantiople seems unlikely.