One of the reasons the original saga is so loved is undoubtedly because of MNOG, and to a certain extent, MNOG II. It was MNOG that really immersed the player into the world of Bionicle and Mata Nui. The Island was vast, and full of different and diverse environments. Exploration was top notch. Even today, fan works still adapt appearances of the island based on these two games. 2004 and 2005 had nothing of this, and I think it's a shame. It was harder to immerse yourself into a world that didn't have much visual and exploration as its predecessor. I mean, sure, the Toa Metru CD was nice little adventure, but nothing in comparison with MNOG.
Maybe not in the exact style as MNOG, but an exploration adventure game of some sort. Exploring the Great Falls of Ga-Metru, or visiting the Great Furnace in Ta-Metru. Maybe walk by the Sculpture Fields of Po-Metru, seeing all the great statues the carvers have made. I would have loved playing that PS2 Metru Nui game. I'm sure it would be a big contribution to the Metru Nui immersion!
Come to think of it, world building was slowly dying out as the years went by. Voya Nui had VNOG. Mahri Nui and Karda Nui had nothing. Bara Magna had that pretty boring 3D fighting game and that's it. I think it's a shame, because Immersion was Bionicle strongest point in the begining.