That would leave a ton of Romanians in Hungary, I could see some radicals wanting to secede in the same borders as 1940, which would be a lot fairer to both sides, Szekely get a corridor to Hungary, Romanians get to keep most of Transylvania, but that's probably a very small group of Hungarians

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You don't see Vojvodina Hungarians wanting to secede, they live in the richest part of Serbia, enjoy pretty much all the same rights as Serbs, there are some that want autonomy like for Szekely, but nobody is seriously wanting to separate, also because most of them have Hungarian passports and can travel freely between the two. It could be different for Szekely, I don't know; countries in the region aren't known for minority rights