Big trouble for Germany. Romania stays stable and a remains a neutral democracy because it isnt sacrificed to Hungary and the Soviet Union. Hungary also stays neutral as you imply. Yugoslavia likely remains neutral as well since they wouldnt see a threat in Hungary and thus no coup of course. Germany and Italy would have a really hard time to invade Yugoslavia. Italy might not even join the axis. So Barbarossa is gonna be harder as well without Romania. But Germany does have more troops since Yugoslavia, Greece and North Africa do not happen. Even heavier Barbarossa?
Wow all that because one man decided to call it quits.
It's gonna be hard to achieve that neutrality when Nazi armies start marching in en route towards southern Russia.
I'm assuming they're gonna be in between, of course.
that or Hitler decides to disturb Hungary as much as possible, trying to get a pro-German regime instead of Horty. If Italy is on board with Germany they have very high economic leverage against Hungary. Support for an armed revolt in Transylvania might be attempted too. However, whether such actions succeed is far from given.
Hungary couldn't it's economy was totally dependent on German trade, so they were screwed. It was also the last official ally left to Germany in May 1945 for that reason (also because Germany was able to kidnap the HoS's son when he tried to defect):
https://en.wikipedia.org/wiki/Operation_Margarethe
https://en.wikipedia.org/wiki/Operation_Panzerfaust
WI after the first Vienna award and Slovak war Horthy does nothing more, and stays neutral when the war starts (no second Vienna award). How would the effect the war and if the war goes like it did in the otl would Hungary become be invaded by the USSR and turned into a communist state.