Regarding the Toa Hagah specifically as they've now frequently appeared in discussion:
Given that pretty much everyone seems to agree that them being Metru builds would be "correct", I would like to point out that this would basically result in a pure 3D modeling contest if it were held as a MOCing contest. There's as good as no way to mess up a Metru build enough to be objectively worse than another one.
Hence in those regards an art contest would be the only thing viable at all in my eyes. And that would probably extent to most Toa. Can't have Varian not look like Norik, right? All Toa Mangai need to look like Lhikan, correct? And at that point it basically would just become slapping different masks/tools on Metru builds.
So yeah, that probably removes at least about half of the viable characters from serious MOCing competitions.
And I've also be mulling over the other potential candidates for canon depiction:
First group are the shapeshifting characters - for example Triglax, Kojol, the unmutated Mistika Makuta, the Makuta of Stelt or humanoid Miserix.
I mean, I admit, I'd probably be able to live with pretty much every depiction of them, but they're shapeshifters, so defining any random canon appearance just because we can is kinda pointless. They can look however someone wants, so why go to all that trouble of defining a specific form of them that was never described in canon?
What could work, however, would be MOCing canon forms these characters took in the lore, so for example Triglax in his De-Toa form or Gorast in her "Blue Titan" form.
So after covering that, what noteable enough characters are even left? Helryx, Winged female Order of Mata Nui member, Lariska, I think potentially one or two Dark Hunters (?), Marendar, Orde, Zaria, Chiara. Those are all I can think of off the top of my head.
Any Toa would be defined a lot by the masks they wear, which I feel would give good 3D modelers an unfair advantage which again means that MOCing contests for them are pointless. The Dark Hunters (except Lariska) I can see happenening. Marendar I can also see happening, but then I'm kinda indifferent to Bara/Spherus Magna lore, so it's possible others might see that differently. The Winged female Order of Mata Nui member is fair game, I'd say. And Lariska and Artakha are likely a problem, because no matter what happens, there will probably be at least as many people dissatisfied with any depicton of them as people which like it.
So that's my personal stance in those regards.
Why I still keep voting no, however, is the allowance of 3D modelled parts, inclusion of CCBS and, after it has been brought to my attention, this:
Just like with CCBS this is a case where there is neither right nor wrong. It is unfair to especially younger people to ban CCBS, yet personally I'm just unable to see CCBS in a G1 character build and say "yeah that'd be fine for canon". Similarly I can see it being unfair to enter an older MOC a lot of people have been refering to as their favorite version of a character for some time already versus a new MOC nobody has seen before. This would make it really hard for newer entries to win, yet if the older MOC wouldn't be allowed to be entered this would also make a lot of people disappointed and angry. Not to mention the whole dilemma with people who have already put time and effort into something they care about which might now not be allowed to be entered. So there's really no right choice here.
And if there is no right choice, and there is no need to make a choice, then isn't the right choice to be against making a choice?