6a. 3D printed pieces are limited to masks and weapons
First off, what exactly constitutes a weapon. Do shields count? What about a Toa Metru build Toa Hagah with custom Bohrok Shields on the chest? Could I use a blade with ball joints as arms? Or blades as fins? Could I put a naked Custom CCBS Torso on a hand and call it Artahka’s hammer then, claiming it’s a weapon piece, use that torso in the Main Moc? What if there were custom ammo for some of the already made weapons (I.e. Cordak, Midak, or Protector Elemental Blasters.) Or even custom Blasters like a crossbow that uses Lego rubber bands to launch axles. What about a catapult on the back of a Moc?
Second, why only weapons and masks? More choices only means even more creativity. As long as it follows the other rules, I think these would only make the contest more fun. There are all sorts of things there could be: Custom bones, armor, eyestalks, hands, feet, axles, technic liftarms. They just add more detail and variation. It’s definitely possible to make good looking Mocs that use these kinds of 3D printed parts and still feel like Bionicle.
Third, there should probably be rules on the piece’s design. 3D printed pieces should at least connect to Lego in some way. Technically someone could use a gun that can’t be held by a Moc. There should probably be size restrictions so you can’t say, put a human-sized mace on Helryx. It should at least feel like it is a Lego piece.