Suit enhancements/extra parts
I was re-reading volume 9 and noticed a picture of Kishimoto wearing an extra piece of suit.
It's one of the two colored drawings included in the volume release, so it might actually exist inside the Gantz universe, but it just hasn't been revealed yet. It might have been one of those scratched ideas (I hope not), but I think it could be some sort of trigger that activates one of those extremely powerful weapon[s] (there could be more than one). It sounds like something out of Zero One, but it could be possible. Now, look at this:
This is one of the drafts included in volume 13. It looks a lot like Zero One and nothing like the Gantz suit, but it could be a draft idea of the super powerful weapon.