When it comes to expansion, make sure your reasons make sense. The Serbia to Yugoslav plan has potential - It fits with western Slavic nationalism and how it has played out in the past. Hungary could theoretically claim Transylvania due to the large Hungarian population there... but it would be difficult.

I would be fine with there being crazy wars all over the Balkans, but none of us want to see simple cut-and-dry expansion. No "Sir, we have invaded their borders and they have retreated and surrendered and we are awesome and perfect and winning." A Balkans war should be bloody - it should tear you up more than it helps you. Prolonged sieges, ethnic violence, terrorism - that is what we should be seeing. Make your wars stalemates without signs of ending. If you do that, it can work.