With a POD of your choice (but the more recent the better) involving the least amount of butterfly effects as possible given the circumstances, make it that in the 20th century both these statements are true:
1) Szeklers in the old Kingdom of Hungary, but especially in Transylvania consider themselves to be Hungarian-speaking Romanians/Vlachs
2) Slovaks in the old Kingdom of Hungary, but especially in Upper Hungary consider themselves to be Slavic-speaking Magyars (let's call this "Tóthok")
(I think this would have solved a lot)
What would such a scheme mean after (an alt) WWI in your opinion?