I’ve actually thought more about Annona’s place in the story. She could act as a hidden villain throughout Brickonicle with her monstrous creations falling from the sky into the oceans of Mata Nui’s Coffin. These monsters could be encountered with their origins unknown to the Toa or the Titans.
Annona herself, I imagine, would basically be a planet-sized ball of Cthulhu (imagine a planet completely encased with tentacles, courtesy of Karzahni’s suggestion).
Annona’s Prison: The Sun, called Annona’s Prison by Mata Nui and His sons, could serve as Annona’s Prison, which is held together by the Nuva surrounding it. The Nuva have doubled as the locks to the prison, preventing Annona from escaping.
After Mata Nui is put to sleep and the six Nuva in the story are taken to His Coffin, the seal could weaken significantly but not enough for Annona to escape. Because the Nuva are placed on a single planet, Annona’s Prison is basically tethered to the Nuva by a chain, keeping her in her place.
However, this connection could be weakened during an eclipse, in which a celestial body (maybe the Moon) could obstruct the space between the Prison and the Coffin, allowing Annona to fight through the fiery plasma encasing her and gaining a foothold. Over time, the connection would deteriorate until it would finally be broken, unleashing Annona and destroying the Sun.
Not sure if this is as good as the first pitch in this discussion thread, but it could add some lore.