A good sampler would be useful, with 16 multiple stereo out's, choke, round robin, probability, at the very least .. can we have that ?
There is a ton of 3rd party samplers all with pro's and cons. useful and necessary. Cyclone GUI .. ewww forget it ... it's too dated.
My fave is Giest 2 but it's unstable, I like MPC Studio / MPC beat .. but that hangs the DAW, Maschine is ok and seems to be reliable, Steinberg's backbone has some awesome features, Satalia is a little unstable. Kontakt is a beast, Serato Sampler is semi useful. Breaktweaker is handy, Nerve is useful, Battery by NI is good.
So an ideal instrument would push creation, be intuitive and above all else .. be stable. Out of all the one's I've tried the microedit and polyrhythm from breaktweaker, the polyrhythm, pattern based filter FX / gate / pitch, transmod and round robin of Giest2 would fit the bill.
Cakewalks step sequencer could just be tweaked to fill this missing role if you design a way to have each note control a set of samples.