Here is an interesting POD in 1886 Prince Alexander 1 Battenberg (Bulgarian prince)
http://en.wikipedia.org/wiki/Alexander_of_Battenberg had to abdicate. In 9 august there was an successful coup against him and the counter coup failed because he didn't get the support of neighter Russia or Germany. But lets say GB decides to back the young prince wanting to increase its influence in the region. There for Battenberg appoints Stefan Stambolov as prime minister.
http://en.wikipedia.org/wiki/Stefan_Stambolov
There isn't a big change (Stambolov ise PM IOTL) except that Battenberg doesn't accept any of his resignations. So Stambolov remains as prime minister even after 1894. His strong anti Russian views and pro western European even further separate Bulgaria From the Tzar in St Petersburg.
After the The First Balkan War which ended after Bulgaria Agreed reluctantly to give Macedonia to Serbia in exchange for more territories in the East. But a for a big surprise to Greece and Serbia Bulgaria Singes a 5 year pact Mutual Defense Pack with GB fearing a second war. GB again wanted to increase its infuense in the Balkans and started seeing Bulgaria as a partner in the region.
Surprise Surprise WW1 AH declares war on Serbia. Russia ->AH. Germany -> Russia. France -> Germany. Germany -> Belgium. GB-> Germany.
And Bulgaria wanting to improve its relationship with Serbia Joins the war on the side of the GB. And sends 100k man on the Serbian front to help against the AH. Seeing a chance to return some of its formal land OE joins the Central powers. Bulgaria has no other option except to mobilize its army for a second time in 2 years and sends 400k To battle the OE. Greece feeling betrayed by GB and seeing the starting success of the CP in 1915 joins the war on the side of Germany together with Romania. But before they can mobilize their Troops Bulgaria with the help of GB Captures the west side of Constantinople (Istanbul). This freed enough manpower for Bulgaria that it was able to counter the Romanian and Greece armies but at the cost of the return its army from Serbia.
In 1916 AH Germany and Italy managed to Brake throw the Serbian Defense and in the end of the year Serbia Capitulated. In 1917 The CP focused their attention in the east and managed to kick Russia out of the war. But the new troops that the USA send every month finaly shifted the favor of war on the Side of the Allice. The war ended in January 1919.
AH was split into tree states Austria, Hungery and Czechoslovakia. The new state of Yugoslavia was formed (Greater Serbia). Bulgaria gained all lands south of Danube(From Romania) Some lands including ( Thessaloniki Solun) [From Greece] Macedonia from Yugoslavia Gifted For the help that was shown in the early years of the war. And Constantinople + the lands around it from Turkey.
Bonus Romania doesn't get Transylvania, Yugoslavia annexes Albania prety little other changes in the war (no Galippoli)
What do you think.