Personally I think if this project were to swap platform we would do well to use something which would let us get some sort of economy of scale happening. Not to mention, potentially allow people to purchase pre-made modules.
I would propose this one.. its a modular open platform which a couple of small companies manufacture kits (also ARM Cortex-M3 as the one Ken linked to).
http://www.ucapps.de/mbhp.htmlhttp://www.ucapps.de/mbhp_core_stm32.htmland there is an OS already build for it. OK re implementing MegaDrum on this rather than just porting to different hardware is a totally different thing, but should be more maintainable / scalable and could open up so many possibilities
http://www.ucapps.de/mios.htmlone benefit down the track I would like to see (from any more advanced hardware) is native MIDI-USB... instead of our current seperate uC to convert. It could save a couple of mS of latency in the chain.. everything counts. Plus its not just a couple of mS, you have so much stuff going on cc messages, multiple note on's triggered at once etc, note offs.. messages surely have to queue up causing more latency. OK so it does not seem to be an issue in practice, but eliminating it can only be good.