Nice idea. There's a sun, and a moon, why not an earth?
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Though now the rest of the planets need to be made...
...
Yeah, never mind, too much work.

Yes!
I had a bit of a desc for her on DA;

a - Her job is controlling the Earth, blessing plants and dealing with continental shifts. She also keeps herself in check with Celestia and Luna, to make sure the planet is rotating properly with the sun and the moon.
b - I actually have a little sea-princess I was working on a while ago, I imagine a cousin or a sibling that, like Luna and Celestia, would work together... and have sibling fights on occasion.
c - I had a lot of Dryad/Fae type ideas while I worked on her. Thus the wooden shoes, necklace etc. She's not a fan of lots of metals.
d - And finally her cutie mark is a representation of the earth, so like the symbol on her neck surrounded by something like a mud-splatter.

Making the rest of the planets would be something I'd totally be all for *A*
Especially if I could make an alien pony.

If she avoids metal, and you've got an idea for a sea-princess, and there's Celestia and Luna, and other planets, there should definitely be a metal Princess, and a sort of 'Guardian of the Wastes' Princess for the various asteroids and bits of space junk (Though she certainly wouldn't call it junk. She cares about every speck of dust in space. Naturally).

Maybe I could even commission something from you once you've done my previous request... Though the things I would have planned would be far, far, FAR too expensive for me to cover...

But if you were to take it up as a side project, a practice thing maybe, that'd be kind of cool...

They all need to be different, though.

Okay, I'm thinking about this too much and coming up with too many possible ways to see it through. I just think it's a neat idea I'd like to see done. Do whatever you like.

Oh man that would be so cool.

I would love to take it up as a side project haha... They don't take too much time either, ponies are sort of a 'stress relief' drawing for me. How can you draw a pony and not feel happy!

Which one should I do first? Probably the sea princess or the wastes o3o I imagine the wastes princess would be one of those "baddies" that aren't really bad, just misunderstood, yanno?

I really don't know which would go first. There's too many to choose from! But I agree on the wastes idea. Makes sense, really.