NOTE: This proposal is also outdated. The current (and final) suggestion is being reviewed by the BSS and will not be posted publicly until they've consulted with Greg.
Three is still at the base, with all ensuing numbers as multiples.
Void is basically a superset of Vacuum, it is nothing, the absence of both good (moral light), and evil (moral shadow). It is to be a physical representation of the moral balance present in every Toa, except for the Toa of Shadow. Shadow is to be the only physical Element that would be unable to exist without its moral counterpart, whereas physical Light would be able to exist in a state of moral "Void." Altogether, Light, Shadow, and Void are to be the Three Fundamental Elements, acting as physical manifestations of the three basic moralities found in all things.
"Poison" is just a placeholder name I used for an eventual Acid-based Toa Element. The idea is to simply redefine the powers we've already been presented with, and cause Greg as few headaches as possible, all the while preserving the main concept of the "numerology theory."
On a side note, Void would be the third mixed-gender Element, along with Light and Shadow, and thus the gender ratio would be 3:3:12, with 3 mixed, 3 female, and 12 male Elements.
The very first thing the GBs created was an Av-Matoran, hence the first MU Element they created was also Light, and not a replica of any already existing Element on Spherus Magna. Therefore we could safely say that they viewed Light as an important Element. Naturally, the second thing they would "create" is Light's diametric opposite Shadow. Perhaps they created a Matoran of Shadow, only to discover that his/her psyche was incredibly unstable, and so they realized the physical manifestation of Shadow couldn't possibly exist without its moral counterpart, a.k.a. evil. I'm not saying they made it that way, rather they simply discovered it, which is why they decided against making Shadow a naturally existing Toa Element, but rather an Element to be overcome, and that is exactly why they imbued the Antidermis from which the Makuta would spawn with dormant Shadow properties. The Makuta were never meant to embrace their Shadow, they were meant to evolve beyond it, as Alternate Teridax did.
As most of you probably know, in the BIONICLE universe, Shadow is not merely the absense of Light, it's its own thing. It's tangible and independant, which is why I believe the GBs also created a third fundamental Element, namely Void, which would be the complete absence of either Light, or Shadow. They understood that these three Elements would be closely connected to the three predominant moralities in the MU good, evil, and uncertainty/"balance." Then, using the number three as a base, the GBs would eventually create six primary Elements, and nine secondary ones.
So in the end, Light and Void would be able to exist as solely physical manifestations, without any positive or negative connotation, while Shadow would only be able to exist as an image of evil, and that particular aspect wouldn't be a design of the Great Beings, but rather a discovery.
A Toa of Void could instill calmness or apathy (i.e. complete lack of emotion, urge, and will) in an adversary, or even an ally, depending on the situation. Say you're getting too worked up about something, a Toa of Void comes along, and balances out your emotional state. Or you're furious at a Toa of Void and you want to attack them, they induce a complete lack of will and perspective in you, so strong that you even forget why you were furious at them to begin with.
In physical battle they'd be able to put a stop to all motion, bring about perfect stillness and make a strenuous situation unbelievably simple. Other practical applications of their capabilities would be absorbing Air, Light, and Shadow, and perhaps even solid matter, though I'm not sure how appropriate that'd be.
A Matoran or Toa of Void's personality would be calm, balanced, and incredibly goal-oriented. When considering a plan of action, a Toa of Void would approach the situation with determination, and little to no emotional investment.
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