My vote goes to the expansion board as well.
In fact, I've been thinking for a while that having a single master board that connects over USB with several expansion boards providing additional features could be an improvement over the current multi-USB method. It's been in use in flight simming for a while now. Flight Illusion and Open Cockpits use such a structure, among others.
For those of us without dedicated racing rigs having only one USB cable would be convenient, and reduce clutter.
It could also benefit the simscreen, as the fat USB cable can be replaced by a thinner and/or more flexible cable(The main reason I haven't mounted anything to my wheel rim).
I would argue not needing a USB interface could also reduce cost, though PIC have it built in so you'd only save on the USB connector(though you'd need something to connect boards together instead).
Of course, I'm just thinking aloud. The current system works well, it's just that it makes for such a mess of wires(Even when using a USB hub).