I really like these designs. Especially how you translated Onua's larger set design into a 2D form. However, there are a few details that seem off.
Pohatu's mask seems strange. After thinking about it, I think it's that the mouth opening is too large. I get that it would help make him more expressive, which matches his personality, but I think it throws off the proportions of the mask a bit too much. The defined nose is also a bit unusual to have on a mask.
Kopaka's shield being a standard Medieval-style shape doesn't seem to fit. It looks out of place. A more stylized shape might look better. Also, is his naginata made out of ice? I remember you saying you didn't want Tahu to have a sword that was actually fire. Isn't this the same thing? I would think it would break easier too, even with him being able to reform it. If it's just colored metal, then disregard that point.
Onua's green shoulders should be actual pieces of armor, instead of the painted on look. I think it would make more sense to have them protrude out a bit, instead of being completely flat like they are now. Maybe just have the whole thing raised up off his body a small bit, so it won't be flush with his arms.