1. Add lines at the corners for better depth perception. And make one of them a scale to show the actual depth from surface.
Adding a scale/grid is a cool idea. But not a high priority.
2. Editable colours for the layers
Improving the look of the layers is on our list. No promises, but we're talking about a "simple" computation for the interior heat (turns out this is really complicated) so it will look molten like you might expect (and more what the 2nd image you provided looks like).
3. Show interior layers with a bit more detail at least for Earth, Mars and Moon
We're showing all the layers that we're simulating and that are editable. Perhaps in the future we may add more granularity to the materials/layer model, but until then it doesn't really make sense.
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In short... All good ideas... 2 will happen, 1 maybe someday, 3 probably not.