With a POD or PODs after January 1st 1900, create a plausible state known as "Hungary-Romania" or "Hungaromania" before the year 1939. This doesn't have to be its official name, though, maybe just the colloquial moniker.
The state should control at least the area of OTL 2011 Hungary and Romania.
For extra points, have the unitary state survive (in some form) until the present day.