So far, I've seen a few proposals for systems of Mask Powers, so I've decided to try my hand at that.
A Mask of Power is created by channeling a small piece of a greater being. For example, the Legendary Masks (Creation, Control, etc.) were created by the Brothers from an elder god.
The everyday masks used by most characters were forged at a shrines and blessed by the Elemental Gods, granting the wielder one aspect of the Gods' heralds' (the Toa) natural abilities. (strength, speed, eyesight, endurance, etc.)
For this reason, these masks are limited to physical enhancement.
A Matoran wearing one of these masks may access one attribute of a Toa. Matoran do not have a natural proficiency in using Mask Powers, and one must train rigorously both to access a mask's powers, and use it affectively.
When Toa wears one of these Masks of Power, they gain enhancement in that area which is even greater than what they already possess. (Basically, Toa strength x2)
(Ideas for) Powers: