What would you say that the bionicle design language is? I’m asking because people liked the winning Helryx because it resembled an actual bionicle set. Then I thought about how I’d make my Tuyet moc for the contest and had this idea of making her a combiner from 2007 sets since that was the year she was introduced and I’d imagine that if Lego did make a set for her, it would probably be a combiner with an exclusive mask and recolors like Irnakk.
In short, the BIONICLE design language is the look and feel of the sets from the original run, ranging from the original Toa Mata to the Stars sets. In a few more words, it requires the use of decently sized, detailed pieces, primarily non-smooth pieces, and avoids excessive detail made by composite pieces.
If you asked me, the most ‘BIONICLE looking’ entry was probably Entry #4, which used a lot more parts with specialized aesthetics. Entry #2 is actually a bit smoother than would be expected out of your average BIONICLE set (even compared to the Toa Mata), using pieces that, while they were used in the BIONICLE run, tend to be smoother than what we’d see around the Toa Metru/Inika. The limbs are a bit smoother than one would expect out of the Mata, even.
But it also doesn’t use any pieces or stylings that are super contrary to that aesthetic. Entry #4’s chain mail design is much more of a MOC aesthetic than a set one; I’d say it’s too parts intensive for a franchise that gave us large, detailed pieces. Entry #5 also has a lot of composite detail, especially in the mask design but also in the torso and waist. Entry #6 is too minimally detailed, the legs and arms don’t use any parts resembling the BIONICLE limbs, or BIONICLE limb construction. Entry #1 falls somewhere in between these two camps, it uses a lot of BIONICLE parts but also has a bit of composite construction. It’s also fairly large for a Toa build; with the thigh construction resembling that of the Titan sets.
While I’m on the subject, I’ll mention that this is also why a lot of BIONICLE fans dislike CCBS, because by default the pieces are smoother than the BIONICLE aesthetic would allow. This is somewhat mitigated by the specialized armor ‘shells’ that attach to them. Conversely, the “professional” BIO-MOCist community prefers CCBS to the majority of BIONICLE pieces precisely because the pieces are less specialized. A lot of MOCs have their own textures the MOCist wants to create, which the traditional BIONICLE pieces might hinder. It’s worth noting that the pieces on Double’s Helryx MOC are all pieces that could fit into that smoother aesthetic, especially the Roborider heads. Those pieces are among the traditional looking pieces I see used most often, along with Slizer feet and Vorox/Skrall armor.
I don’t have very much to add from what @Hawkflight said but I want to share my thoughts on how you can achieve that bionicle design:
The main trick is to use more detailed parts and experimenting with combing these pieces in a way that it works and creates a whole. Essentially the best way to replicate that style isn’t neccesarilly to use prefabricated parts but instead working under limitations and use this to create something stunning. If you look at the old Dh and Rahi builds you’ll see that all of them are trying to push the system to its limitations and that’s essentially the style of early bionicle a lot of experimentation of a limited pool of intricate and detailed pieces.