Peter’s Pride
As Peter II of Yugoslavia, overthrow your uncle and become King.

Now This is Getting Childish
As Yugoslavia or Bulgaria, be at war while both countries are ruled by a child.

Dracula’s Revenge
As Yugoslavia, establish the autonomous region of Transylvania and have it own all Romanian cores.

The Balkan Powder Keg
As Yugoslavia, start the Second World War (i.e. be part of either the Axis or the Allies and at war with the other faction in a war you started).

Huge-oslavia
As Yugoslavia, occupy all your neighbouring countries (Austria, Hungary, Romania, Bulgaria, Albania, Italy and Greece).

With historical focuses on, Flag of Yugoslavia Yugoslavia will have Peter II take the throne with the Underage Monarch trait. The Bulgarian focus "Fate of the Balkans" immediately after "Form a Regency Council" allows you to get a puppet war goal on Yugoslavia. Probably bugged under 1.10.1 - "Form a Regency Council" does not trigger the leader change correctly.
 Bulgaria: Regency Council is the current country leader