Given the limitations of having to make these masks on a minifigure scale as opposed to CCBS scale, I think the simplified designs are perfectly justified. At least each of them resembles what I personally believe to be the best iteration of each mask, regardless of whether that iteration showed up in G1 or G2.
Concern regarding the possible fragility of Tahu's mask has already been brought up, and I definitely see it being a problem around the mouth area. Minifig scale is probably too small to split the cheek gaps into two, so it'd probably be best just to make the existing ones a bit smaller to beef up the area around the mouth a little. The shape of the eyeholes might be just a little... aggressive, almost disdainful, but at times both those qualities could be ascribed to Tahu's character, so that works. The design on the torso looks epic as red characters tend to, but Tahu's lack of any visible accessory compared to the other Toa kind of bothers me. Maybe he could use one of those over-the-back sword holders that we see in Ninjago all the time, complete with some golden swords to accompany his flame blade/lava board weapons, kind of like how Tahu 2015 carried his arsenal?
Would I prefer Kopaka's eye color to be ice blue? Yes. Is there a good reason to make it yellow? I guess so. Nothing worth crying over. The idea of his scope-eye being scarred is particularly clever; it could be used for some interesting backstory stuff relating to Kopaka's personality.
Definitely seeing Master Gali in G3 Gali's mask, which I highly, highly approve of. The half-skirt thing echos a lot of varyingly humanized artwork I've seen of Bionicle characters over the years, which works fine. Her color scheme could maybe use a little more contrast between the primary and secondary blues, but I can't say that I know of anything that would look better and I'm sure that's been explored already.
I'm not sold on Lewa's mask. It looks like it's taking mostly after the 2001 Miru, which I would agree is the best-looking version of that mask, but it's lost so much of the... attitude that that mask portrayed. I always saw the distinguishing feature of the 2001 Miru to be its almost manic smile, which I don't really see translated here. I get that the jawline was cut back a bit to make it look more feminine, which looks fine, but I don't get why the eyeholes were opened up to the point where the mask takes on this 'perpetually surprised' expression. I think it'd look better if it the eye shape was narrowed a bit, not quite to the point of the 2001 mask, but certainly less open than here. I feel like that'd channel Lewa a bit more, regardless of gender, unless there's other character changes in play that I'm not aware of. Other than that... if pink accents have been tried and didn't work, oh well. Blue works.
Pohatu's probably my favorite; the printing around the waistline with the spikes/throwing knives, if it could be properly executed on set, looks great. The mask seems to take after Uniter Pohatu's a lot, though that may just be a product of softer shaping, and I much prefer this to the Uniter mask anyways.
I can't call myself a fan of the Axl body that Onua uses as, in my opinion, it takes the proportions just a little too far, but that's what we have to work with so that's what it's going to be. The mask looks 'heavier' in design than the others, which is appropriate, and the color scheme works great.
General comment: at the end of the element debate, the idea of referring to the Toa and villagers as Toa/people of a region, as opposed to an element, was thrown out, like Tahu being Toa of the Volcano. Has that been dropped in favor of just calling them by the elements again? Just saying, if Tahu is now the "Spirit of Fire" that's co-opted/taken over Jaller's body, I still think calling him the "Spirit of the Volcano" or "Spirit of the Core" would sound cooler. Plus tying the spirits to the landscape might make the island all the more of a character, depending on how the spirits and the island are related to each other. Gali could be the Spirit of the Tide...