Ideally, I think that Akira Ifukube would have done a masterful job on a BIONICLE soundtrack. He was always innovating with musical thechnology, and is wholly responsible for one of the most recognizable sounds in cinema (with the possible exception of the lightsaber activation whoosh or a TIE Fighter's screech).
Unfortunately, however, he's a bit dead. So that would complicate matters.
My next choice (one that's actually alive, last I checked), would be Kow Otani, the composer for (among other things, I'm sure) all the music for the Heisei Gamera trilogy and Godzilla, Mothra, and King Ghidorah: Giant Monsters All-Out Attack.
After that would be another (living) composer, Michiru Oshima. The woman responsible for the soundtracks to Godzilla 2000, Godzilla x Megaguirus, Godzilla x Mechagodzilla, and Godzilla x Mothra x Mechagodzilla: Tokyo SOS.
As for American composers, Mark Mothersbaugh completely overshadows anyone else (including the Japanese ones I listed prior [except Mr. Ifukube, of course]) in terms of what would be good for BIONICLE. In case you're not familiar with his work, here are two pieces of music composed by him.