I think transylvania could also join Hungary. In my opinion Wallachia and Moldavia should join into Romania first, then Transylvania should get a decision. It could stay independant, join Romania, or join Hungary. After that I think Romania should have an option to invade Hungary if Transylvania chooses to unite with HU, and vica versa.
Allow me to quote myself:
The communist revolution in Hungary would be the perfect opportunity for a Romanian uprising in Transylvania, and with military support from Wallachia and Moldova they would gain independence.
Transylvania, which is mainly inhabited by Romanians and has just separated from Hungary, has no reason to rejoin it, unless of course you bring some good arguments.