ehm, perhaps a bit late to note.. but I just thought of something that would be nice to have on the board:
Buffering the inputs independently so that two mics could be directly connected to the inputs without the need of an external buffering.
Atm, when you connect two mics/piezo's, they start influencing each other, so they both need to be buffered independently before you connect them to the inputs. So it would be nice if it would be possible to connect these without adding an extra buffer circuit, though I'm not sure this could be done with the current stereo input as it has the common ground, which is probably also the reason the signals should be buffered first.