@White_Noise: had to look that one up, too:
https://www.midi.org/articles-old/midi-polyphonic-expression-mpe
It’s a MIDI controller, albeit with polyphonic aftertouch. If you’re a keys person, that’s probably really neat.
Having played around with Model15 on the iPad, I can see how the Roli Seaboard could really come in hand with a polyphonic synth, esp. one that has a Mod Matrix sensitive to aftertouch. I think the Seaboard has at least vertical position sensitivity, if not xy capability. Such a controller would easily give you access to timbral changes while playing, which is cool.