Hey, just because some new FE has shown up, don't think the love is gone! You've made good progress with NBB and the new version is coming along great. With FE's, the more there are, the better they'll all be.
As for customization, I'd permit custom backgrounds to be inserted, along with car logos.
An easy way to create themes and maybe even a way to bundle themes together with the window transitions so that, for example, "blood orange" encompasses both the color schemes and window transitions.
Add the option for spoken messages to be given by the Mac. Bundle those together into themes. Someone should be able to build a BMW theme, or a VW theme, or whatever. Those themes would include background graphics, fonts, colors, logos, window transitions, and spoken commands.
Question -have you followed a model where the interface elements are disconnected from the actual code itself? I forget the name of this model, but its the Apple recommended on that means you can build a nib file and as long as you name all the controls correctly, you could substitute it for the existing file and have a whole new look and feel.