It will be Gon.
Probably Gyro. The "king's" currently rebuilding his empire somewhere.
Granted I have no idea what happened in the recent Hunter movie, but him turning into the big bad would be a good reveal and explains his disappearance, underground dealings, etc.
His last surviving clan background, smarts, red eye powers, relationship with Gon and others would make for a really good big bad at the end.
Problem with Kurapica is that his hatsu is unusable against anything that aren't the Spiders. I guess having 100% potential is pretty swell but we're talking about potential, not actual output. So coupled with the short activation time of Emperor Time and associated problems/fatigue, it's likely that due to limited training time, his Reinforcement, while trainable to max potential, is nothing like a dedicated Enhancer's (except against the Spiders, but i attribute that to his hate; plot device, etc.) So his only real weapon is judgment chain but he's not chain jailing anyone anytime soon. In other words, he would make an OK villain at best.
We probably won't be seeing any recurring villains anytime soon. Most of them are either too weak or too likeable. I think an effective Villain must also be a somewhat distant one. Yet we've seen the world through the eyes of Hisoka, Illumi (to an extent), the Spiders...can you imagine any of these guys suddenly becoming the end boss again? It would definitely take some maneuvering to make it work.
I think Kurapika will have a new Hatsu later and among all the characters Kurapika is the man who could never be a villain IMO,he likes his friends too much and because of his tragic childhood he really despises injustice.Also he has a certain code of honor.
I guess Gyro will be the final villain.
Hisoka can't be a final villain,he has no real ambitions,but he could be Illumi's ally.
Illumi could be the final villain too,he seems to plan something with Alluka,and there is still what Silva said about Killua in the Zoldyck arc.I think Alluka is the key person who could turn Killua into a villain.
And there is also Gon,I want to see his foreshadowed encounter with Gyro...
the last villain will be someone who you will never expect
Gon will turn evil and be final villain for World Tree to fight
Geez, Nensense, way to overreact. Maybe his post wasn't the most thoughtful, but that doesn't make it right for you to be so rude.
Honestly I don't who might be called a final fillain in HXH, but I admit that Gyro would be interesting as one.
I'm thinking Gyro.
I'm thinking Hisoka for Gon (and possibly Killua's) final story villain. But I don't foresee Hisoka going out in a big battle against Gon or that (nor would I wish for it).
Illumi is likely Killua's final story villain
Kurapika has the spiders for his.
I'm thinking Tonpa for Leorio.
But I think Gyro is going to be the major story villain.
Overall, the amazing thing with Togashi stories is that you have a lot of grey characters. It's hard to tell who's a "villain or not". I don't really consider Hisoka a villain even. He goes around and does as he pleases, without regards to morals or that.
I'd go with Gyro. Gon and Hisoka can have an epilogue fight all they want.
Gon going dark is also a good option.
Ging could also make a great Grand Puppeteer.
Either way, the big bad starts with G Even though G should mean Good.
Discontinued in 20~25 years after Togashi's death. Still at that time without a glimpse of what would be the story's conclusion.
The strongest villains so far always were the long hiatuses, Gon and the others fought them so hard they nearly died every time. So yes I'm gonna call it, the final villain will be an endless hiatus