I don’t see how Gravity is “earth-based”.
If I were to “rearrange” the Elements, I would aim to have 12 Elements in total (2 groups of 6), meaning we’d have to get rid of three Elements.
Contrary to popular opinion, I would actually keep Earth and Stone (and Iron, while we’re at it) separate; I feel like there’s enough of a difference between the two (three) to justify separation, and it keeps any one Element from being able to control such a majority of the universe.
Water and Ice are another pair that I’d consider merging, but each of those Elements branched out over the course of the story, to the point that they aren’t as similar as their names would suggest; “Ice” is more like “Cold”, and Water has additional healing properties associated with it.
As for the Elements that I actually would get rid of, I agree that Fire and Plasma can be merged; we never really saw an Elemental Plasma wielder, but what little we know about them overlaps heavily with Fire, especially since, similarly to Ice, “Fire” kind of just became “Heat” as the story progressed. If the Element of Fire was more restricted, Plasma might have a niche to fill, but as it is now, it seems kind of redundant.
The other redundancy that I can see is Magnetism and Iron. I can’t think of anything that a Magnetism-wielder could do that an Iron-wielder couldn’t unless we get into electromagnetic laws, and even that is such a specific range of uses.
I can’t really think of any other two Elements that could be merged. I guess I agree with this assessment here:
In theory, I don’t have an issue with Light as an Element, but the story always treated it more like a power associated with The Good Guys than an actual Element, so I’d either get rid of it, or more clearly differentiate “Light”, as the ability to control photons, from “Light”, as the moral goodness inside most Matoran Universe beings.
EDIT:
This here gave me an idea:
Chat with Greg Farshtey | Page 305
Keep “Light” as just a reference to moral goodness, and then the Element that we currently know as Light could be revamped as Radiation, and expanded to the non-visible spectrum.