Well, plenty of
potential things. In strictly defence terms, they buy weaponry from US and Germany. Hungarian air force operates Swedish Gripen fighters and thus most likely relies on Sweden at least for spare parts (which military jets need lots of) so here's another pressure point. They are big beneficiaries of EU funding (not a NATO thing but this is all intertwined) and already at odds with EU rules, which could push them towards not getting the funding - there are plenty of reasons to implement that already, just lacking political will right now. They can find themselves sidelined for new EU spending, political nominations, and so on.
The question would of course be if EU/NATO want to do that. It's part of the usual political horse-trading game.
For sure, in the extreme scenario if US/EU/main partners really wanted Finland and Sweden to join and Hungary vetoed, it could be made to pay an unaffordable price.