In my opinion, many Hero Factory masks are not very good for BIONICLE MOCs because their designs often include decidedly sci-fi features like antennas and microphones that would not make a lot of sense for a BIONICLE mask (though, to be fair, the original Akaku Nuva had a microphone molded into it as part of its design as well). Of course, some of them DO avoid this issue, including one of my favorites, Breez's helmet from the Brain Attack series. That one also benefits from being a neutral color, meaning it could ostensibly used for a character of any element. In fact, most of the Brain Attack masks (with the exception of Rocka's) lack the obvious electronic gadgets of previous versions of those masks.
A lot of the Brain Attack villain helmets could also be used for BIONICLE villain or creature MOCs, since they already tend to have a semi-organic look that might be appropriate for the BIONICLE universe. Frost Beast's in particular seems somewhat BIONICLE-ish in its design, as is Dragon Bolt's. In fact, most villains' masks might work better for BIONICLE than the hero helmets, since few of them have obvious electronic gadgets attached and since BIONICLE villains explored a very diverse range of face designs.
i've thought about this a lot, actually. a number of HF helmets/faces do look decidedly Bionicle-eqsue, and i've considered coming up with names and powers for them, as a sort of fun little fanon thing. i actually just got Frost Beast today and was struck by the face pieces's potential as a Kanohi--an "immoral" Makuta one, say.
I was just thinking about how some BIONICLE fans think that Hero Factoy "killed" BIONICLE and I came up with an idea to possibly change their mind
as much as i'd like to think that this isn't still a thing four years after the line's launch, it is. but don't think that you have to change their minds. the people who are still angry at Lego for "killing" Bionicle and replacing it with Hero Factory are kidding themselves. they are, to my knowledge, a distinct minority of the Bionicle fandom and you really shouldn't pay them any attention.