You mean, what happens if Bulgaria defeats the combined forces of Serbia, Rumania, Greece, and the Ottoman Empire? Probably they would conquer the entire Balkans and Middle East since for this to happen alien space bats would have had to have eaten the entire non-Bulgarian population of the region.
Bulgaria is only likely to win if they have outside intervention in their favour, which of course would mean a general war in at least Eastern Europe and a French Front somewhere if Germany thinks it needs to get involved obviously.
A-H intervenes to save Bulgaria and Russia consequently intervenes to to save Serbia in that case. Now whether it stops there really depends on Kaiser Bill, if he has to intervene to save the Austro-Hungarians from their foolish gambit to save Bulgaria in the first place.
and so you probably have WWI earlier but with a different orientation perhaps.
If Germany and A-H are at war with
Russia over Balkan and Ottoman influence
the position of Britain and Italy would be interesting. Italy probably won't intervene until the British position is established. If Britain is neutral then Italy may still be convinced by France and Russia to enter.
the Ottoman's are on the Allied side and the relationship between France and Britain is good so would there be a reason for them to enter either way?
The straits would be open so Russia is probably supplied with what it needs this would probably still lead to a CP defeat and a less destructive WWI. The US and Britain if neutral would probably both favour the Entente powers.
Probably no immediate breakup of the CP empires though as defeat would probably come quicker, or a negotiated settlement after the futility of the fireworks becomes apparent for all to see. So in retrospect..I take it back..definitely ASB. Bulgaria probably cannot even win with outside intervention. Unless of course if its a peace of exhaustion with large swaths of Russian terr. still occupied and France and Italy making little headway against Germany or A-H. then you might get a negotiated settlement with Bulgaria at the what? San Stefano borders and the Enos-Midia line? Somewhere the straits would have to be closed to choke the supply of war materials to Russia and I don't think thats possible here unless Britain were to find a reason to intervene on the CP side and accomplish a successful Gallipolli analogue. Highly unlikely though not impossible. The Balkans and Eastern Europe are devastated and trench warfare has still taken its toll along the Italo/Austrian border and the Franco/German.