I think, conceptually, one of the things we're really trying to step away from is a character that is the embodiment of evil, or the embodiment of good. We've seen that story told in Bionicle already, and it really ends up falling short narratively. I know some people are attached to something like the embodiment of chaos and evil - ala the Joker, but with the restrictions that a kid's LEGO line is going to have, portraying that isn't going to be something that we'll be able to do. Plus, coming off the Makuta and Karzahni storylines, I feel like it's a solid step backwards in terms of conceptual villain or hero characters.
My argument with Takua was that we'd be better off not using the character at all, or if we were, we would need to directly address the subversion his role in the current story. In our first draft, the kid was unnamed. Naming him put this new emphasis on the character, he would have to have some more significance than what we were doing with him. Mata-Nui as the (dead) All-Father I think properly fulfills that reference.
(I would also actually say that Mata Nui isn't as important a character as Takua in G1, but that doesn't really matter)
The major thing with this pitch that I'm seeing is that while there is some really cool moments and character designs, there's not a lot about how this ties into the other arcs thematically, or how it impacts our main characters and their own personal arcs. If the spectacle outweighs the characters, the actual story will get lost in the weeds.
Even the new characters don't seem to have layers complexity as actual characters, but rather as cannon fodder to drive the final siege home as being vast and epic. To shape the pitch a little better, I think you guys would need to pay more attention to those details.