You make a good point, but couldn’t the powers be used for the characters to add some depth to them, or establish some kind of conflict for them to get past? Like, for example, I would imagine Toa and Matoran of Stone tend to be very stoic, unchanging, maybe, since Stones tend to be very hard, solid masses; meanwhile Toa and Matoran of Earth are more ‘grounded’ [pun not intended, I promise] and behave in ways similar to how the Earth might [its a bit hard to explain] as the Earth is more of particles, dirt, sand, and such; its always changing, if slowly, as do the Onu people. In short; the Po People often refuse to change, to the point where they have to be ‘shattered’ if they’re to change, while the Onu People accept change and are kind of like the zen people, while the Po are strict and stuff.
This could potentially put the Onu and Po people into conflict with each other [on a sub-level, perhaps] where the Po are often irritated by the Onu and their more ‘chill’ and accepting ways.
To go a little further, this could create conflict with Pohatu and the Po people as he’s more light-hearted and friendly, and even more willing towards necessary change, almost like the Onu people; ironically making the hero of the people an outcast of sorts. As for Onua? I’d see him as appearing slow, the most motion he does casually tends to be tunneling, which he does when he needs to think long and hard; he’s also at something of a conflict with the Onu because while they are accepting of change and that it is inevitable, they prefer natural change, while Onua prefers to change things himself; that’s why he tunnels, because its a change in the landscape he can control.
Also you could say that in a backstory the Earth people did in fact control both Earth and Stone, but they were too powerful and ultimately had to be divided into the two Elemental classes.
Anyway, those’re just some ideas. The original material doesn’t need to be altered just because on the surface it might not make sense to some people; in real life there are reasons for the tings that don’t at first make sense, as such, I think it would be beneficial to have material that encourages the audience to look beyond the surface for a better understanding [that sounds almost like something my interpretation of the Onu people would say…] rather than alter it for immediate convenience [which is the mistake that G2 made]