I would not find customizable button layouts to be an aspect of themeing and don't feel it should be incorporated as part of a theme system overhaul. As well it seems that most of the developers feel that this is an unwanted feature in Rockbox at this time. (While this may be debated, it should be noted that you're very unlikely to change opinions and doing so might waste time better spent on the following topics of discussion.)
Improved theme control is something that could definitely be considered a potential positive in Rockbox, but it's unclear from your detailed description exactly what it is you intend.
What do you want to add to the while playing screen?
What do you want to offer in the way of options for customizing the menus and the filetree?
What exactly do you consider "global options" that would necessitate a new theme language as opposed to enhancements of the existing one (and a new language for the menus, rather than a new language for the WPS as well)?
Have you looked at the existing WPS code and considered whether your ideas can be done with the current drawing routines, or require reworking of significant other systems (for example how text scrolling is handled) in the process?