Very good. I think I'll take my own swing at a pitch.
I think I'll deviate from the just a little bit from the SU formula.
During the insane confusion of Brothers' return, the Toa discover a mystical seal around the island containing the chaos, a last line of defense erected by the Gods to prevent the Great Powers from wrecking havoc on the world.
In order to pursue Makuta who has circumvented this shield, the Toa must break it. But, breaking it is beyond and normal Toa's power. So, they must learn to "purify" their "divine power," and "pool" this power, to form the Kaita, "the Elemental Gods' champions," who wields the summery of all previous Toa's power and the heroic experience.
These forms could not be maintained for very long, so the Toa would still be present in the story, but they could be summoned for a few big, important moments.