How about making it easier to make themes by creating the source image programmatically? In effect this would be a new theme creation process:

ThemeMaker: create theme template ->
Gimp/Photoshop (optional): edit generated source image ->
ThemeMaker: edit theme details ->
finished theme package

You could have selection of different theme colours, edges/corners on bars, textures for bars, gradients, etc. in the first phase.

The first phase could be implemented with alternative software (scripting Gimp?), it would give more power for us who can program, and it would allow people with little Gimp skills to create their own themes or just prototype a theme idea.