I've not yet received my Model 12, which I'm looking forward to pairing with my MPC Live as an interface in a two-way affair (ie, recording to and from both, and of course using each in isolation at times, in combination at others).
Based on what I've seen and read, the main feature on my personal wishlist would be the addition of takes per track, to allow for bouncing and storage. I use this feature a lot on the DP24, for example bouncing all my keys /guitars/whatever to a stereo group file, hiding the individual takes, then reusing the original tracks for something else.
I can of course effectively set this up with the MPC storing takes as separate samples, but it would be great to have it built in to do it natively.