One of my ideas is to grow Yugoslavian influence, either through war or diplomacy. Like creating a balkan alliance with Romania, Bulgaria, and Greece, or just mercilessly conquering them all to create a single Balkan realm.

One Focus you could put in Yugoslavia would be to make a pact with Italy and germany, a pact that gives Italy Dalmatia and Germany Slovenia so that they can be no war. That is if both Germany and Italy accept Yugoslavia's offer. Also a good Focus may be to make a Turkey and Greece join the war if the first focus I just mensioned doesn't work. Both of does focuses can work in my opinion. :) Good luck.

There should be branches focused on the Comintern and cooperating with the USSR, building up defenses on the Italian and Austrian borders, or forming a Pan-Balkan Empire with fascism and allying the Axis (with Blaze's idea as well).

Considering the Balkan's general abundance of Aluminum boosting Support Division research would be nice. I always try and base my army with what I have available (for example Turkey has more heavy tanks and heavy ships due to the amount of Chromium in the area).

I too would like a Balkan empire/Faction for Yugoslavia. The order of Foci should be (in my opinion), Albania, Bulgaria, Romania, Hungary. Also having Focii to have pseudo-meetings with Germany, Italy, and the Soviets to prevent intervention in the area would also be nice. Post annexation having a Focii or two to jump start the fortification of the area would also be nice.