I've been thinking about this since I got into a balkan wars binge a while back. What if Bulgaria just didn't declare war and start the 2nd balkan war?
From what I saw it was a really close thing. They feared that if they did not act immediately, they would lose Macedonia forever. In retrospect we know that 2 more big opportunities arose, but even back then there seems no reason to think the end of history is at hand.
So if they bode their time and peace'd out at the end of the first and only war, maintaining reasonable relations with their neighbors, less destruction overall, and slightly larger borders especially against the Ottomans, what would happen next?
If they joined the central powers in ww1 it would probably end up similar just with those Turkish borders maintained. But the entente offered to pressure Serbia into giving up some southern land in return for Bulgaria joining them. OTL it wasn't enough to convince them and Serbia refused giving more, but with less bad blood Serbia could offer greater concessions while Bulgaria would be more willing to accept them. While it could not be expected before the end of the war, Serbia gaining all the south slavic lands within AH could offset even giving the whole of Macedonia up.
Also, with how much closer Bulgaria's borders are to Constantinople, assuming Ottomans still enter, the entente would be much more interested in getting Bulgaria to join up. If all of this happens, maybe the Turkey carving would be far more successful. At least they would definitely get thrown out of Europe.
There seems to be no good maps of the borders before the 2nd war were declared so you'll have to forgive me for using a tigerstar video screenshot.