Honestly, I came in here REALLY late, but better late than never, I suppose.
Personally, I feel like we should just keep the Toa the same, with the exception of Pohatu. I honestly think he'd work well as Iron (or metal, or whatever), partly due to the fact that Po-Matoran culture has a lot to do with crafting and creation. Likewise, the material people usually think of when it comes to creation is metal- It's used in some of our greatest innovations and technology, and is far easier to work with than stone. You make one mistake on a stone sculpture, for example, and you might have to restart with a new block.
But with metal, you can just remold the stuff as you see fit, and always reuse the material as one sees fit. It's something I can see Po-Matoran use. And, yes, I get metal was also something Ta-Matoran worked with, but this was when they were Mask Makers in Metru Nui, and since we seem to be sticking with the island of Mata Nui, I think it's a change we can work with.
Not to mention, Iron and metal in general can work in a desert setting. Metal can be found in canyons, so I imagine that an Iron Tribe would live inside a canyon within the desert, using the shade to their advantage and having huts and villages sticking out from the cliffs, with mines built in to harvest iron from.
Also worth noting is that the natural color of iron ore can also be a bit of orange hue- One of the colors for Iron Toa in G1. As we all know, Pohatu's worn orange before (albeit to mixed critical response), and bronze is also another color for Iron Toa in G1, so we could use that if we gave Pohatu control over metal in general.
Not to mention, I don't think Iron would be TOO different from stone. Instead of Pohatu lobbing chunks of stone... we'd have him lobbing chunks of iron. Instead of making all the stone in the Shadowed One's fortress crumble, he'd make all the iron collapse, etc. It's not like changing Kopaka to Lightning or anything- While not exactly the same thing, Stone and Iron are still a lot closer together than most of the element changes (such as Jungle-Air).
That being said, I'm also completely fine if they keep the elements and cast the exact same, so long as they don't give Stone the short end of the stick, like they did in G1... and G2 for that matter. Really, the main thing I want to see in the end is more developed and human personalities for the Toa, and that's really it. Anything else I can work with (besides getting rid of one of the Toa entirely).