1) What if Bulgaria is allowed to win, no Austrian ultimatum to stop? What does Bulgaria gain?
2) What if Serbs did not start the war, either because of less interest on their part, or because they do not get encouragement from Austria?
3) Can Bulgaria lose? Can this lead to Bulgarian losses of territory, or the Bulgarians being obliged to give up Eastern Rumelia? Maybe Ottomans get in to take back Rumelia, and the Rumanians invade to take southern Dobruja?
On the one hand, Bulgaria is much bigger than Serbia at the time.
However, I get the impression Bulgaria's move to expand into Eastern Rumelia was unpopular with the great powers, even though it turned out to be successful. In OTL the Ottomans did not like it, the Austrians seemed to oppose it and backed the Serbs, but the Russians also hated the Bulgarian King.