Have Bulgaria get the borders shown in the picture above by 1990. Also have Bulgaria become communist following WW2.
What if Bulgaria manages to avoid the Treaty of Neuilly sur Seine. Bulgaria just is not part of that war. Perhaps then Bulgaria can take Turkish Thrace in a joint war with Greece on Turkey. Turkey could maybe get a seperate peace deal with Bulgaria, and then defeat Greece. Or Turkey could be unablet to defeat Bulgaria een if they defeat Greece. Or Turkey might lose the war with Bulgaria and Greece. Perhaps Bulgaria would want a population exchange with Turkey to consolidate it's terriory in such a scenario.I can see a bigger Bulgaria but with most of the Straits while Russia would never share it...?
To have the borders of that map you might have to start WWI earlier with no Great Power intervening and the Ottomans even more controlled by retarded Pashas. The Pasha commanding the forces in the 1st Balkan War was a failure. Make him fail more.
Another thing is Russia being in dissaray when WWI and Balkan Wars happen like in OTL 1917.
With all that... you want it to be communist? Stalin OTL prepared for war against Turkey in 1945 to get the 1878-1914 border and possibly the straits. If Russia still turns communist and thus takes Bulgaria with it then Stalin will make any land accessing the Sea of Marmara a Soviet territory. And Bulgaria is in no position to oppose.
With all asb-ish luck possible to get those borders but if Bulgaria turns Communist you won't have those borders no more. Not entirely. This is not even including Greece who will refuse Bulgaria to have their final destination and will fight for it which in result will draw the Ottomans/Turkey back in war.