Well, I hope not too late to say that...
Since it is a application, it might work under an operating system, so does the interface.
Plugin are not that necessary because there are apps can replace it(but I think some are OK).
I've download the Android alpha version of Rockbox, and using for about two month. It's OK, but the interface. It looks like an OS running on a virtual machine, rather than an application.
I think it will be great if we can use the native interface on the OS. Rockbox can be written as a "kernel" of music player(Console UI), and the graphic UI can send "commands" to the kernel. The interface can be written separately from main Rockbox project by someone who is good at designing interface. While the interface is depends on native OS, themes won't be needed! And, the porting would be faster.
--> Windows, Linux, and Mac version won't be a simulator anymore!!