This list turned out to contain items not directly related to .xml support, but most of the entries are
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1) As for the multiple rings/belts, allow more than one such entry per body in xml.
2) Also, it would be fantastic if the belts could have a full set of orbital parameters to set, or at a minimum just inclination
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3) For the rings, it would be nice if we could specify a custom gradient to use as well as a way to set the size.
4) External model support in the form of allowing model files to be in the user media folder, and a way to specify a model without having to edit appearance.xml - much cleaner that way.
5) Re-enable the normal mapping option if possible
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If you do, make it specifiable directly from the system.xml
6) About the lighting, I think the easiest thing would be to set a generous radius (maybe half a light year or so) inside which there is no falloff whatsoever. It may not be realistic, but a bunch of dark outer planets aren't much to look at either
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We can just think of it as exposure compensation. The basic amount of light emitted should of course be directly related to <Luminosity>.
7) If possible, fix the density problems that arise when exploding objects.