So as I see, there is a microcontroller core - old PIC based and new LPC and STM32 discovery based.
Is it really comfortable to use discovery boards? Haven't you thought about nice PCB with place for MCU (as STM32F103 one)?
I wonder if any part of discovery board is used except the microcontroller - thing is STM32F4 cost locally about $35 and the chip itself is about $12.
If the schematic for STM32F4 core board is tested, I can make the tracing for separate MCU and share it. Same thing is for in/out, analog, midi and other modules you connect to main board - haven't you thought to use SMD components, place them at both layers and make PCBs really small to fit into a portable device? (because I'm seeking for it at least) - like AKAI APC KEY 25.
So if such thing is interesting - I can do that.