Have Greece join the Central Powers. Then the Entente gives Bulgaria a free check to rape Greece to pieces. Throw in some territory in Marmara as well (Erdine and Kirklareli province sound good) to keep the Bulgarians satisfied with minimal gains in Serb Macedonia. Also throw in some mention of Southern Dobruja.
Serbian-Bulgarian tag team would probably be able to hold Austria-Hungary and the Ottomans off indefinitely as long as they're getting support from the allies. Probably knock out and occupy Greece as well.
Romania then becomes key. Whether they join the CP or the Entente (latter is more likely imo) it opens up a huge front to Serbia and Bulgaria's north. An allied Bulgaria (and surviving Serbia) might be able to prevent Romania's embarrassing defeat in this tl then.
Austria-Hungary is gonna be hurting due to having to keep the Romanian and Serbian fronts open longer.
In the Middle East, the Ottoman Empires might find themselves in some trouble in the Caucuses and the South as they throw more forces at Bulgarian defensive positions. Bulgaria's too close to Istanbul for them to not take priority.
End result: Extremely good for the Entente. Romania and Serbia might avoid falling. Austria-Hungary has to overextend itself even more fighting and bleeding out on four fronts. The Ottoman Empire faces difficulties as well. Bulgaria is happy. Serbia is happy more or less. Greece is... well, Greece is fucked. Italy, Bulgaria and Serbia are all gonna want pieces of it.
Post war Turkish war of Independence might be fun. There's no Greece, but the frontline of the conflict is now Istanbul depending on how committed Bulgaria is to the idea of Istanbul as an international zone.
Not sure about Russia, they're probably still doomed to revolution.