The USB DAC idea is absolutely something I'm planning on, it's actually probably the first module I intend to design. :-) BlueTooth was an idea you came up with, but is an awesome one that is added to my list.
In short, anything you can get line level analog out of and fit on a 1.200"x2.500" PCB is possible. The 1.200" sides are presented on the front and rear panels, so there's not a lot of space for UI, but enough for a knob or a couple buttons and LEDs. I love the idea of a USB player, but the UI would be the limiting factor there.