Hmm. ISTM that a big knock-on area is the Turkish Straits. After the First Balkan War, Turkey-in-Europe was reduced to two enclaves: Istanbul and its surburbs, and the Gallipoli peninsula. If as OTL Bulgaria remains neutral at the start of the war, while Turkey joins the CP, ISTM that a Dardanelles operation has a much better chance of success. Or maybe not? I don't know that any reinforcements or supplies reached the peninsula overland and through its very narrow neck.
Another important knock-on is what happened inside Turkey as a result of the Second Balkan War. Turkey had been grossly humiliated in the FBW; in the SBW Turkey recovered significant territory, which I would guess got them back off the floor. They no longer had unfriendly troops almost in artillery range of the capital. This campaign greatly boosted Enver Pasha.
If OE joins the CP, Bulgaria will jump in the entente ASAP to try take Constantinople( wet dream) so... gallipoli?
let's say Churchill still tries to take the peninsula but the young Ataturk still is able to resit: Bulgaria joins the entente on the condition to take Constantinople( demilitarized or half granted to Russia so their are happy as well) and rushes into the capital entrenching a good portion of Turkish army in the peninsula and entering the capital in no time. The OE is basically defeated and Russia might be able to resist with english supplies from black sea.
If Bulgaria join entente, Serbia will resist and Austria will hope for Romanian intervention because the whole Bulgarian army( 1 mln and 200k at full mobilization) is coming to save the Slav brother from the evil German.
Even if Romania joins the 2 Balkan brothers would be able to resist thanks to english supplies and by 1917/1918 try even an offensive( Bulgaria would be a monster in te Balkans if fully supplied with general vazov), Austria would collapse sooner and Romania alone would surrender.
Peace conference same as OTL but in the Balkans:
- Romania looses all the dobruja area, even northern to Bulgaria
- Serbia gains Croatia/Bosnia/Montenegro/Slovenia as OTL but gives macedonia to Bulgaria as thank to help
-OE explodes but Constantinople goes to Bulgaria( full control if USSR happens)
-Romania and Hungary would pay all the debts to Yugoslavia and Bulgaria.
interwar:
Yugoslavia faces one less nation problem( Macedonians don't exist, they are full bulgarized) and maybe has a bit better economy, and haves good relations with Bulgaria for the first time in centuries( one saved the other and the other, same religion and culture).
Bulgaria is living a golden Age as Third Bulgarian Empire and might experience a population boom, in 1922 the Bulgarian/Greek war starts( instead of Greek/Turkish) and Bulgaria smashes the greeks( Balkan Prussia with 4 times size army) and gains Salonika and seizes the Greek navy, basically controlling all the claimed lands since the second Bulgarian Empire.
The great depression strikes as OTL but Bulgaria is able to recover faster than OTL and doesn't go pro-nazi but stays pro-France/England making friendship treaties with Yugoslavia and maybe recognize the USSR to avoid as old friendship( Russia freed Bulgaria).
Greece goes full revanchist toward Bulgaria and might join axis, if Benny doesn't try a coup or invasion.
ww2:
starts as OTL, Hungary and Romania join sooner the axis and let's add Greece.
Bulgarians and Yugoslav use the Danube as barrier on the north and alps on east while smashing Greece( even if Italians help the troops of both nations would be poorly equipped, and Bulgaria has a stronger navy while Greece lost it all), if England seizes control of east Mediterranean faster they land troops in Constantinople and go help( in the mean while Croatia has upraised occupying half Yugoslavia).
Greece goes down by 1941( no supplies and surrounded on earth and sea).
1942 lither is loosing in Russia and Balkans( no Normandy since the Balkans are already in allies hand and is closer to Berlin), Romania looses and Hungary sooner follows.
1943/1944 lither is dead, Allies reach first Berlin( few days before Russians).
Peace( only the difference, if not mentioned is like OTL)
-Germany splitted in 3 ( usa, England,france) looses Austria but keeps Prussia( Danzig back to Poland)
-Poland, Romania and Baltic to Russia sphere
-Bulgaria seizes all the macedonia area from Greece
-No Tito in Yugoslavia(gains Albania) or communism in Bulgaria( Allies members)
modern day:
-Bulgaria is a monarchy/empire(Tsar Simeon II) and would have maybe 14 mln people( 9 in 1990 OTL 7 in 2016 because of communism), good economy and is a nation leader of EU.
-Yugoslavia is a federative monarchy( king of Serbia and duchies of Bosnia, Croatia, Slovenia,Montenegro and Albania) with some unrest( it's the Balkans after all)
-Macedonia nation concept never existed.
-Greece is a small European nation with worse economy( more debts on Germany

)
-Germany never experienced communism nor the wall and has even better economy.
thoughts?