So, as we all know, the Mata Nui legend was crucial to G1. It helped to introduce us to the world and main conflict, and provided a backstory for why anything is actually happening. What I initially misinterpreted about Bionicle, though, was the nature of Mata Nui and his relationship with the Matoran. I'd assumed that he was actually their god, and that they worshiped him as such.
With that in mind, as well as the all-important worldbuilding that we all so clearly value (and with good reason), here's a little thought I had about spicing G3 up a little: give the Matoran an organized religion.
The benefit to this is that you can tell a lot about a culture by their religion. Take the ancient Romans, for example--while the Greeks saw Ares as a force of aggression and malevolence, the Romans venerated him as Mars. From this, we can infer that the Romans thought more highly of war than the Greeks. Such is the same with the Matoran. If, in the reciting of their creation myth, they proclaim the 3 virtues to be outright divine in nature, and that following them is the path to Artakha, while defiance leads to Karzahni, then we can say that they hold those virtues to great importance.
Additionally, this may make it easier to keep it ambiguous as to whether or not Mata Nui truly existed, and if he did, what he was like. This adds yet another possible mystery, keeping in with the trend of mysteries set by G1. The religious nature of the background also makes the Rahkshi cult make a bit more sense--they're an offshoot of this religion that has gone in the opposite direction and worshiped the Matoran devil.
But hey, that's just my 2 cents on the Matoran culture and a specific aspect of worldbuilding.