I know a lot of people were pretty bummed after 2010 when Tahu ended up being officially stripped of his Nuva status and left that way--especially when the other five Toa Mata retained their Toa powers--meaning no more Akamai or sharing Nuva mask powers or anything. The way Journey's End and the comics are written, it seems evident Tahu himself was quite hurt with the devolution himself, and it's hard not to sympathize with him, as the narrator makes no secret of Tahu's opinion on the matter:
--“What have you done?!?” [Tahu] raged. “The battle of my life, and you reduce my power?”
--Tahu said, still bitter over the mask’s actions.
--“That cursed mask,” Tahu said, in a tone of barely controlled fury. “It’s ruined me.”
Nor is he ever shown reconciling with or learning to accept this loss. Instead, as it stands, it seems that Tahu as a character ends the saga he's been at the center of stripped of the hard-earned Nuva powers destiny had provide him with.
However, The Yesterday Quest refers a few times to "Tahu Nuva," (one of which refers to a character who would not have known Tahu during his time as a Nuva, nevertheless creating an image of "Tahu Nuva"). Would you say that the Ignika could have restored Tahu's Nuva powers (as it was making Aqua Magna's inhabitants amphibious and generally rebuilding Spherus Magna?)
(You'll have to forgive me if this comes across as being somewhat passionate, but the writing makes it seem like a tough move to pull on him as the journey ended, and I'd at least like to try and help return him back to his status quo)