1) Clip could have a Key & Scale defined, so that all the notes falling outside of it are hidden and cannot be chosen. Alternatively, clip could follow a global Key & Scale in a manner of Chord Track known from Cubase or Studio One (that can be faked with Multi-Note, Diatonic Transposer and Note Pitch Shift, but it's really painful to set up).
2) If you'd select a part of clip - a vertical slice, a rectangular area - you could generate random notes following the Key & Scale and perhaps other settings like likelihood that notes overlap (legato), density of the pattern, variation in terms of velocity, length, etc. You would also be able to select existing notes and alter them as above.
3) You could select a single note and drag it up to create a chord (again, following specified Key & Scale) so the longer you drag it the more notes are added and - with modifier key - you'd change the form / voicing of the chord; you could drag it to the sides to create arps or strums, with additional options to finetune them.
4) You could add conditional triggers to notes, not unlike in Elektron devices, so simple probability of the note getting triggered but also more deterministic stuff: only trigger when previous note was triggered (or only when it wasn't), trigger it every 2nd time the loop plays, trigger only on 1st or 4th loop play (great for rides or breaks), etc.
What you think?
Check the video of how Reason's PolyStep works:
https://www.propellerheads.com/shop/rac ... sequencer/
https://www.youtube.com/watch?v=4Xo4YmEGlxY
) it would be very cool if they gave the piano roll some real love & attention. 
