Alright, so obviously I and many others have gone over the rather lacking MIDI implementation, but there is one aspect that I don't see much talk about. Which is the ability to use an external MIDI interface (or more than one for that matter). This is really equivalent to using external MIDI devices, so there is not really any difference just because it would be an interface.
There is MIDI over BT (bluetooth), WiFi and USB3. Strangely enough they only allow inputs for all of them, so you are left with only the 4 MIDI Output Ports for MIDI data (on the X). Again, to be standalone, sure I'll buy that, but a studio centerpiece needs more. So what I'd like to see is them adding USB3-MIDI Output Ports, so that a class compliant USB3 MIDI interface could be used with the MPC. THAT would rock.
The extension to this is naturally that we need the 'multi-timbral' (multi mode in Akai speech, according to Dan) MIDI inputs to be able to organize all of it.
Anyways, another topic that MIDI gets involved in is automation.
Currently we have an insert dialog for MIDI data, one message/event at a time. I mean it works, but it is so painful to use. Why not have a range insert dialog, with curves and shapes, etc., because adding single messages really only works if you use things like Program Change or perhaps hard panning (only Left, Center, Right) but even that, when you want sounds bouncing, would be cumbersome to add. The dialog does not remember any parameters the second time around, so you got to edit every single parameter each time. They could put an intern coder on a range insert dialog. Enough MIDI for now.
Ok, so what about using the 4 sample layers in various modes, the original (like it works now), but then add the ability to mix two (or more I suppose) in other ways than just literal 'layers', say cross (a'la 80's Roland)/ring modulation, or amplitude modulation. Both are relatively cheap, in terms of processing. Now imagine this implemented in addition to the current layering parameters and so for example velocity would impact the modulation outcomes. Could be a very interesting addition, for relatively cheap.
Another thing I've been wanting to explore, is a multi-mode filter effect, where the output values (e.g. LP, HP, BP, N) would be mixed according to some algorithms and spread across the stereo field. This would generate a spatial effect that could be made very snazzy.
I've tried to pick things that are either atomic (can be developed on its own) or what seems to me as possible to implement with relatively small changes (except for MIDI, but that's because basic aspects are MISSING, and not features, IMO) from my POV. I may be wrong. And there are nicer things that would be cool to have, but seems more involved in terms of the MPC.