Well. the 8 outputs would probably be enough for many so what would be needed is just more A/D channels. AFAIK there are no multi-channel ADC's available anywhere but there are many multi-channel DAC boards (miniDSP, DIYINHK etc.). Also there is a slightly better AKM ADC chip available, the AK5578EN which is pin-compatible with the AK5558, and it's not much more expensive ($10 to be exact). However, if the PCB manufacturing cost is about the same for bare A/D vs. combined board it doesn't really matter. Just wondering the need for the FPGA or DSP as the XCore processor is quite powerful, on Diystompboxes there is another Xcore project where one is being used for cabinet simulation and guitar fx (http://www.diystompboxes.com/smfforum/index.php?topic=114354.0), also there is software for ADAT I/O.