Well lets see:
Illumi could easily end up as final villain material, if he manages to take control of Alluka or killua (or both) to get his wishes granted, only problem is, he has no army like Gyro or a troupe like chrollo.
Gyro is most likely final villain material, but I still wonder how will things work out for him, if that happens.
Chrollo at this point doesnt seem like he will be a direct final antagonist, he keeps losing spiders left and right, I´m thinking he will end up as a ally to the final villain most likely Gyro, unless he finds out about Alluka´s nen, then all bets are off.
Pariston also has good odds of being final villain material, if he gets dissapointed with the hunters association that is and Ging knows he´s a big threat so he wants him monitored, that just goes to show Pariston is a big deal also he´s a very good psychological fighter, which I think will be togashi´s main focus point with his final villain, it wont be a about a overpowered physical monster, but about a psychological monster, I´d like that concept more actually, it would be a bitchslap for many shonen out there to have the final villain of a series like HxH be more of a psychological fighter.
Hisoka and his final fight against gon, seems interesting, but I doubt Hisoka will turn into final villain.
I´d pick Gyro or Pariston for final villain material, but more Pariston.