This list turned out to contain items not directly related to .xml support, but most of the entries are
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
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
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
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.