The GSR mystery feels, to me, like a Journey to the Center of the Earth kind of deal. So G3 would need a similar world-centric mystery that also reveals something about biology and the origin of life on the surface. Another example is the planet in the Discworld novels being a flat disc balanced on elephants atop the back of a giant turtle- which isn’t a mystery, rather, it’s a world origin- but it has a similar feel.
For any Bionicle revival to have a mystery equivalent in grandeur and thematic relevance to the GSR reveal, Faber probably needs to be involved since the GSR mystery was his idea inspired by his own life. His art for both 14B2020 / “Biovival” and Waveborn have already developed intriguing mysteries such as “What is hidden in the ice beneath the surface of Antarctica?” and “What is found at the bottom of the ocean?”
G1 was about a man composed of machines taking medicine to save his internal ecosystem from a cancer. Faber’s new project or any hypothetical G3 could have a similarly concise thematic pitch; Men composed of machines planting seeds and exploring depths to survive in a world that has already died. maybe we aren’'t told that the world is already dead, and that’s the big mystery. On the other hand, it’s a very pessimistic mystery. Maybe a better one is there is another world on the other side of the one the characters know, and they just have to dig through the Ocean floor or travel over the edge of the flat Earth to reach it.