Okay, so I've been pondering recently what the "Helmet" slot should actually be in the game. In many cases, there's no reason for any of us to ever wear an actual helmet or hat of any kind, and if they WERE made available, they could easily fall into an accessory slot, of the two everyone has.
So instead, I've decided upon "Limiter" as the slot use.
As far as I can theorize, Limiters would do as the namesake, and limit your character in one way in order to accomplish something more worthwhile in the long run. This could include additional battle experience, mana cost reduction, or gaining additional stat points upon leveling up.
Really, as long as the reward isn't an immediate numerical boost, there's a ton of possibilities. Feel free to come up with a few.