I agree with this. It'd be nice to see songs that skin things like the slider ball track colors, spinner circle, etc. It would also fix problems custom numbers of frames for the slider. For example, in the two skins I'm working on, one has 12 frames for a slider, one has one (only because I haven't animated it yet). People probably would skin 10 frames by default, so they'd see two frames of skin 1 per second, and only see a static slider in case 2.
A similar issue I've had, that may not be fixed by this, is with the animated life bar in my GGXX skin. If someone reskins the health bar, the color (the animated part, that is) won't be overwritten because the file name is different, so it looks all goofy, as such:
Perhaps the single frame life bar (and the few other animate-able elements) per skin could override the animated elements of the user's chosen skin? Just thought I'd throw that in the mix, although it's not technically related to the topic.