Early on in the Zoldyck mini-arc (when Gon was rescuing Killua), Silva mentioned something along the lines of Killua coming back. That's why he let him go with Gon. That intrigues me a lot.
This is how I see it. Killua is the most fragile of the four emotionally (at least when it comes to the people he cares about, like Gon and Alluka). I'm afraid of the time he might come to a breaking point. I'm seriously dreading if Killua will ever come to this point. I don't think he will become an antagonist, but I don't know what will become of him. And this scares me.
Kurapika is harshest of the four, in the sense that he's an asshole who will stop at nothing to get his revenge (as long as his friends don't get in trouble through his actions). What's scary about him is his ruthlessness and directness. He has an iron will, which is both admirable and frightening. I don't think I need to expand on this since this has already been discussed, and we will see much of this in Yorkshin anyway.
Gon is the most dangerous of the four because... well, for a lot of reasons. But mostly because of his mentality (that I will not explain here lest I derail this thread further) and recklessness. And this goes Up To Eleven when he's emotionally disturbed (like when he was all RAAAAAGE over Pitou). I've already said I'm scared that he'll die, because there's a huge possibility that he can at any moment. If he doesn't get killed by circumstances or other people, he might destroy himself.
Leorio is the most normal and stable of the four. I love him. If there's anything for me to fear for this guy, it's that he might not appear as much in the manga or the anime

. It's still my dream that he gets his own spin-off. Something like his clinic days or something. I don't know.
I just realized that I could be more scared of/for the main characters than the myriad of antagonists in the series. Interesting.