I was thinking they would have a gauge with 4 elemental bars. Each time they used an ability of a particular element, the bar would grow a bit. Whenever they filled the bar, they would be able to use a more powerful ability of that element. I was thinking about the armors (perhaps even buffing the potency of the armors as more of an incentive, depending on the rate of generation). The amount of elemental energy contributed to the bar would depend on the frequency the element would be used. For instance, Water abilities would be used commonly, therefore they'd provide less energy to the bar, while their air abilities would be used less often, thus providing more energy each time they're used. This particular mechanic would almost force them to use a variety of elemental abilities in order to use the armor from that element.

To be honest, the gauge is a bit more challenging of a concept than I thought it would be. I would like for it to have synergy with their game-play -- something that requires the use of a multitude of abilities. I want it to force them into being more than a turret healer. My imagined design was more of a support class with healing capabilities -- able to cast heals and help out, but primarily worried about buffing and dealing damage when possible. Thoughts?