1227 (as OTL), Theodore of Epirus and Thessalonica drove the Nikaian garrison out of Adrianople and annexed much of Thrace. Bulgaria and Nikaia allied against him, defeating and capturing him and dividing his lands in 1230. One competitor on the Balcans was removed. But in 1241, Bulgarian czar Ivan Asen II died, and Bulgaria lost its influence on Serbia and Epirus. Still, Bulgaria defended better than OTL against Nikaia in 1246, kept most of the conquered (former Byzantine) areas, had to cede only Adrianople and Athos and help Nikaia against the Latin Empire.
Since the 1240s, fewer German settlers went to Bulgaria and other eastern European countries, because the ravagings of the Mongols never had taken place.
In 1257, Constantine Tikh I was elected new Czar of Bulgaria. After the difficult years following Ivan Asen II's death, he gave the country more stability. There was no revolt of Ivailo the swineherd in 1277, Constantine Tikh I continued to reign. But after Constantine's death 1280, Macedonia became independent. In the following decades, the states of Epirus, Byzantium and Bulgaria would compete for this area. Michael Asen II became new czar.
But the biggest threat was yet to come. In 1310, the Rum-Seljuks defeated Bulgaria the first time, taking the South with Philippopolis (Plovdiv) and the Aegean coast. Czar Michael Asen III fell in battle. 1319, the Rum-Seljuks conquered the disputed area of Macedonia, and in 1329 there was another Seljuk victory against Bulgaria. Sofia fell.
But during 1353-55, Kay Khusrau IV of Rum attacked Bulgaria again, took the capital of Tarnovo. Karvuna suffered the same fate 1360 when it was made a tributary of the Rum-Seljuks.
Under Seljuk rule
1384-90, the Seljuk lands were struck by Black Death, which reduced Bulgaria's population too.