In Mask of Light, there is one line spoken by Makuta that seems like it sets up a mystery. He says, "Sleep spares him [Mata Nui] pain. Awake, he suffers."
That's interesting. Nobody ever thought there could be a reason for Makuta to put him to sleep besides a power grab. What pain does sleep spare him? What else happened to Mata Nui to cause this pain?
But nothing happened. Nobody ever alluded to that line, no pain of the sort mentioned comes up. (I suppose Mata Nui's worsening condition could count, but that wouldn't make much sense for him to allude to, especially since he was dying BECAUSE he was put to sleep iirc.) They slapped that mystery on the table and future story turned its nose up like someone had just cracked a rotten egg there.
If bonesiii was asked about this line, he'd almost certainly handwave is as "makuta lies" (and if he sees me call that handwaving he will almost certainly give me a diatribe about how it's a fallacy to call it handwaving because he cannot be wrong) and if you asked greg, he probably wouldn't even remember the line, and likely either say makuta lies, or even strike it from canon. Which would make sense, because canon did jack diddly squat with it anyway.
I wish they had done something with that line. That had the potential to pull Makuta from the depths of Angry Janitor With Lazily-Written Plans Land and maybe even dropped him into the Domain of Interesting Characters.
But they didn't. Oh, dear.