Sorry folks, this is gonna be a long one:
First of all, I agree with everything in Nitro's comment, especially the part about masks. It just makes much more sence to focus on preexisting parts like G2 masks, Hero Factory Helmets, Throwbot and Roborder heads, and parts from other themes, withing reason, of course. If you wanted to use K2-SO's head as a mask, you'd have to use it n a creative configuration, but you could still make t work, and there are HF helmets in their base-forms that just scream G1 Kanohi to me (such as Core Hunter and Stringer). Also, @Planetperson and others, if the concern is fitting the masks on G1 heads, just use Knights Kingdom visors- they can fit on a Metru head.
And, if you want to avoid bringing those pieces into canon, you could just build your own masks from preexisting small parts. Masks built this way always stand out to me as cool and creative.
This s assuming, of course, that the masks will be decided by MOC contests at all. I personally favor the use of art contests to make Kanohi designs. Or, what if we made Kanohi contests in the style of the G2 Mask of Creation contests, where MOCists can create them large-scale? That could give us some fun, new designs that would stand out from existing masks.
(also, if there's ever a contest for a Skakdi, it would be a shame not to use the Black Phantom helmet with brick-built teeth.)
@Gilahu, about CCBS: You and many others including Messo have said that you "value the G1 aesthetic". But I ask you- what aesthetic? If you compare Tahu '01 to Tahu '10, I would argue the two feature different aesthetics by definition, since they depict the same character, n the same form, in two different styles. I think that's part of the beauty of Bionicle: seeing the hodge-podge of different characters in different styles come together on the same plane. I would say that the Toa Nuva feature a completely distinct aesthetic from the Toa Inika, but in canon they're both valid. I think it would be a shame to discount someone's interpretation because it features fewer pistons than a MOC from 2011, since, to be frank, it's no longer 2011. Design sensibilities have shifted, and I think that the Bionicle canon should reflect this, not deny it.
I will say that @Gonel makes by far the best argument for limiting post-bionicle parts, but I would say a good compromise would be to not use the identity-centric pieces in the same way as the previous sets.For instance, I understand frowning upon the use of Hero chest plates and Shadow Traps being used in the same way as their original sets, but it would seem silly to limit their potential as creative armor and weapons.
Also, allowing Ben-10 parts will at least be essential to making a canon-aligning version of Artahka since Swampfire introduced so many sand-green parts.
@Mesonak, question: you've stated that this will only be for characters with speaking roles. This raises the question: what about Rahi? There are many species that effect the plot (especially during the Metru Nui years) that never got depicted (such as Trollers, Tunnelers, and Lava Eels). I would hope these would get depictions to, no?
Also and @Eljay may want to chime in on this one, I feel that pre-mutation Baraki would be significant, since they would provide us representatives for what are, canonically, the 'six prime species' of the Matoran Universe. Not only would this fill in some slots on the wiki, but it could also inspire a whole slew of new MOCs that would use these images as creative spring-boards for species formally to amorphous to make anything 'official-looking'. I think that they would encourage creativity rather than quash it.
As for only allowing 3-D printed Masks and weapons, this just seems silly. People will inevitably use masks as armor (think Luker) and incorporate 'weapons' into the structure of their builds. That's the beauty of Lego and Lego-compatible parts: you don't have to use them for their intended purpose. As stated, I don't think 3-D masks need to be involved anyway, but if you do allow them, you should go all the way.
Finally, I know that it isn't what this poll is about, but I' like to throw my voice into the course that suggests an Order of Mata Nui contest, a another time, perhaps. That, I think, would breath even more life into the community than these competitions.