Know Nothing
Alternatively have a 1914/15 POD. A slightly better organised Gallipoli campaign plus approaches to Bulgaria has it side with the allies, or at least declare war on Turkey. [The monarchy and many politicians were biased towards Germany but some argued for joining the entente powers].
This helps force the Turks to concede E Thrace, most [barring the straits themselves] going to Bulgaria. Germany and Austria might declare war on Bulgaria but can't get to it without going through someone else.
Bulgarian entry also triggers Greek entry on the allied side as they don't want to lose out or be isolated. This enables allied forces/supplies to reach Serbia, which along with the fact Bulgaria isn't attacking it's eastern border means it survives.
At the end of the war, which is probably 1816/17 at latest Serbia becomes Yugoslavia but it's heartland is exhausted after several years of war and the new northern territories are deeply hostile to coming under Serb control. Bulgaria has secured it's SE flank and has received military and economic aid from the allies [part of their price for joining the conflict]. Therefore in a subsequent conflict, aided by Croatian and Bosnian Bulgaria easily defeats an exhausted Serbia and grabs the territories it feels belongs to it.
This leaves the areas it disputes with Greece and that lost to Romania in 1913. Not sure about the latter but possibly also a war with Greece which is long a conflict with the Turks in Anatolia.
It needs some luck but is possible. What is more difficult is probably holding all this territory afterwards, with so many disgruntled neighbours.
Steve
I see problems with this. One big problem here is that getting Bulgaria to join the Entente is like getting Italy to join the Central Powers; more of the land desired is controlled by the side in question rather than the side joined in our timeline. Bulgaria did want as much of Thrace as they could get, but they wanted Ohrid more, and to weaken Greece. As much as the Italians may have wanted Savoy, Malta, and Nice, they wanted Trieste and Trent more, and wanted Istria if they could get away with it, but had designs on Dalmatia more broadly. Needless to say, most of these areas would make an alliance with the Hapsburgs essentially unpalatable.
Furthermore, the circumstances you present give Romania further cause than merely Bessarabia for joining the war on the opposite side than was true in our timeline. Here, southern Dobruja and Bessarabia are reasons to momentarily overlook continued Hungarian control of Transylvania. In essence, any aid that the Entente gives Bulgaria, if we accept your underlying premise, will be blown in a war with Romania, and not used to help save Serbia.