I listened to BS (no pun intended) #18, and on the heels of the Eldritch being/therapist concept, I approached this character from a different perspective.
This pitch retains Annona’s status as a dream-eater, but what if she also became a scary story to make Matoran children go to bed?
“If you don’t go to bed, Annona’s coming to get you.”
However, this begs the question: How do the Matoran know about Annona if she’s a being from the far off beyond? Suppose, in the early years of Artakha’s creation, Annona discovers this newborn planet, and—as indiscreetly as she can—begins feeding off the pre-sentience Matoran. They do nothing about it because they have no will of their own, and it becomes part of the reason why Ekimu gives them sentience. Then, when Ekimu throws her out and is ready to grant sentience to the Matoran, he gives them a word to never take lightly: Annona. The Matoran have no idea what the word means, what it is, or even who warned them against it. All they know is that if it ever comes back, the nightmares won’t cease until it leaves.
Unfortunately, as part of the developing atheism, the Matoran have since disregarded her, turning her into nothing but a story.
On the other hand, one can’t really blame the Matoran, as Annona hasn’t gone anywhere close to this planet since.
Maybe she’s merely biding her time, waiting for someone—someone very important—to go to sleep.