I've noticed that a couple other users are posting their thoughts on what the ideal elemental roster would be, so I thought I'd contribute. I'll explain why anything from G1 was changed (i.e. why some elements were cut) as I list them. To avoid taking up too much of your time (and also because I'm lazy), I'll try to keep this brief and not go into how this affects worldbuilding except where I feel it's necessary. If that is wanted, however, I am willing to provide via a lengthy Google doc. Also, the given color schemes for each element are pretty flexible, so Gali's secondary color could be silver one year and orange the next.
These are the elements utilized by the main cast, and the ones most prominently represented among the Matoran populations.
-Fire (Ta- prefix, red-and-orange color scheme)
Yeah, most of these elements will be self-explanatory. The only thing worth talking about here is the fact that Fire doesn't denote heat energy or chemical combustion, but the flame itself, which is actually just hot gas ("hot air," as they say). This enables us to lump in Plasma, which is good because it wouldn't be that functionally different from Fire to start with. So yeah, Plasma's canned.
-Ice (Ko- prefix, white-and-blue color scheme)
Ice stays separate from Water on the basis of its differing properties and practical applications. Ice is more stabby-stabby, Water is more splashy-drowny-millionsofdollarsinpropertydamage-y. Ice is also a bad conductor.
-Water (Ga- prefix, blue-and-silver color scheme)
I have literally nothing of value to put here. It's just water. Case closed.
-Earth (Onu- prefix, black-and-green color scheme)
Ok, so the Earth vs. Stone topic has been a subject of debate as of recent (for which I'm probably responsible), so I'm gonna address just why I'm lumping Stone and Earth together. Basically, if Stone and Earth are separate, that leaves only dirt for Earth, which doesn't make for a very cool element. Additionally, since one of the goals for G3 is to make Bionicle easier to get into for newcomers, the fact that most people will expect Stone to fall under Earth might pose a problem. So Stone is put under Earth, and replaced with...
-Iron (Po- prefix, reddish-brown-and-silver color scheme)
Why Iron? Well, the idea's inception had something to do with the image of a desert with iron in the sand, which would make it look like the surface of Mars--a red desert. The image is both fantastical and lets Pohatu keep his sand powers, which are cool.
Now, the issue of metal not naturally occurring a whole lot in nature has been brought up, to which I say this: why can't this fictional planet have much higher natural quantities of a metal that doesn't oxidize (which is a thing, such as with platinum, or even some sort of made-up natural form of steel)? Because then, the issue of naturally-occurring, pure metal being needed to make Iron unique as an element is solved. Pohatu can just tear metal from the ground, and boom, he has something at his disposal.
I've also heard that the motifs of Po-Koro's culture might be different because in G1, it revolved around stone sculpture. But Ta-Koro didn't really have much to do with actual fire; only a volcano, which just spews molten rock. Why, then, can't Po-Koro get away with something similar, and keep its sculpture?
-Air (Le- prefix, green-and-keetorange color scheme)
I don't see why Air needed to be changed for G2. I understand Lego was going on "muh focus groups," but it didn't make sense for the elemental roster, and it didn't fit the world better. Le-Koro works as the village of Air because it's physically very high up in the trees and, by extension, the air. So yeah, the wind-flying stays.
-Light (Av- prefix, gold-and-white color scheme)
Whether this is included in the "main cast" depends on what direction G3's story would take and whether it would include Takanuva. I can see him as a mentor figure who trains the Toa. Not an old man, though, but something more like his G1 counterpart; I like the idea of a sage figure who isn't an old coot.
-Shadow (Kra- prefix, black-and-red color scheme)
So of course, this is the "bad guy" element; however, it's not actually an element, and here's why: to specify what Shadow would actually be, it would be pretty much a vacuum where no matter or light is present. When struck with a blast from a Shadow-wielding being, the damage comes from the vacuum within, not any physical or energy attack. Because it's not any real substance or energy, it's not a true element, and no Toa have access to it.
These elements would be established early-on, but no major characters would wield them unless they're introduced in future story arcs.
-Lightning (Vo- prefix, cyan-and-purple color scheme)
VORIKI CONFIRMED BOIIIIIIIIIIIIIIIIIIII
In all due seriousness, Voriki should totally be a character. The element's colors are even purple for this sole reason; though whether they should stay that way is up for debate. Also, I'm lumping in Magnetism with this one because A. that leaves room for other elements and B. it reduces the potential redundancy of Magnetism to Iron. Also, depending on how Matoran brains would work, Psionics could be lumped in here, too, since it needs to go anyway.
-Plantlife (Bo- prefix, dark-green-and-brown color scheme)
What? I said I didn't want plants to replace Air, not go away. I disagree with Var's idea of lumping Plantlife in with Earth for two reasons: first, because plant powers have a number of different applications from earth powers; and second, because plants are not a part of the earth, they just sort of grow in it. Heck, some plants may take root in rocks, and some live underwater, so couldn't they also be lumped in with Stone or Water? Plantlife is a fine element; it should stay where it is.
-Gravity (Ba- prefix, purple-and-gunmetal color scheme)
I know this one might be a bit OP in practice, but the actual effort needed for a Toa of Gravity to use their powers in an OP way could be really high to compensate. I think Gravity works as an element because, according to the Theory of Relativity, gravity is but a result of the bending of spacetime; so a Toa of Gravity is really a Toa of spacetime, which is very much a real thing in nature. At least, it is according to advanced physics.
-Poison/Acid (Ke- prefix, yellow-and-black color scheme)
Ok, so this one is a bit of a weird choice, so let me explain myself: I just think it's weird that there's no "poison" element. I think a Toa who can control corrosive liquids (i.e. acids and bases) of impossible PH levels is a neat idea, and one that should be at least considered. Just a thought, nothing more.
-Kinetics (De- prefix, tan-and-blue color scheme)
The idea of Kinetics as an element has long been discussed by fans, and I think it's a valid option because it could really have some unique applications. Making targets stop or start moving, loading up an object with potential energy and them hitting it so it goes flying--that kind of thing. It's not like it would be hard to get kids to understand, either; all you have to do is say, "He's the Toa of motion." It would at least be unique, that's for sure. Oh, and since sound is moving particles, Toa of Kinetics could replace Sonics, which I think should get the boot because it's not a part of nature so much as a phenomenon.
-Magma (Su- prefix, black-and-orange color scheme)
Alright, I just really wanted 6 secondaries, so here's the best I got. Water and Ice are separate for the same reason this is separate from Earth. Plus, I wanted an "orange" element because, I dunno, marketing.
So yeah, that's all I have to say. Leaf a like and shrubscribe if Plantlife is your favorite.