Greetings, folk of BZPower! I bring you a subject on which to idly speculate...
It seems reasonable to assume that the original decision to give BIONICLE characters magical masks was inspired by the ritual use of masks in real-life indigenous cultures. In such cultures, however, masks do not merely grant power of one kind or another. Rather, they allow the wearer to temporarily embody or become the god, spirit, legendary character, or revered ancestor whom the mask represents and play that being's part in a ritual performance.
Now, it is certainly the case that canonically, kanohi do not have this property. But...WHAT IF THEY DID? Imagine an AU in which each Kanohi is named after a god, deified hero, or other mythological entity (and the Matoran have a pantheon extensive enough to provide all of these spirits) and designed to visually resemble them. To use the Kanohi's power is to channel/incarnate the spirit temporarily and thus have access to their power (which is the same power canonically associated with the Kanohi). What might this pantheon look like? In other words, let's have some fun coming up with spirits whom different masks might represent! I imagine this would work well in one of those reimagine-bionicle-characters-as-humans AUs that folk sometimes come up with, in which characters don't necessarily wear their masks all the time.
I'll start us off with a few & probably post more later (feel free to come up with interpretations different from mine or to expand on mine):
Hau = Obviously some sort of courageous warrior god who provides protection in battle. (Charms and amulets to protect against enemy warriors' blows are actually really common in real-world cultures, too.)
Kaukau = A spirit of the sea, presumably. Or maybe the central figure of an Earth-Diver myth.
Miru = A bird spirit, given that it looks sort of "beaked" as well as having a flight-like power. Maybe the Kadin is the same, and its spiky mohawk represents a crest of feathers.
Mahiki = A shapeshifting trickster god.
Rau = The culture-hero who invented language.
Huna = A shy fairy or goblin that always stays hidden from the sight of mortals. Would fit with Vakama's initial timidity.
Komau = A guardian of divinely ordained law, maybe? Or, almost the opposite, a tricksy enchanter who could put spells over the minds of others. Or maybe a legendarily persuasive leader/rhetorician!
Zatth = A Mother of Monsters type figure or spirit of the wilds.
Crast = A legendary monster cursed to be hated by all beings, to the point that even inanimate objects recoil from it in revulsion.