I have a few other ideas that might be really useful for live input, particularly for the situation of playing a one-note (or one-octave) left-hand with a soloing right hand:
A mode where the root note determines also the mode of the chord (e.g. you tell ARP you're in the key of C major, and you play an E as the root note, the resulting chord should be an E minor as that's the correct chord for the key, etc.) -- a lot of times sampling the root and then some random lead note above it gives just one or two notes to the list that ARP picks from when playing over it, and that's a little frustrating. This would work really well with one-finger and split.
A mode where the notes are only "sampled" once per beat, so you can noodle away furiously but hit the chord you want on the beat without messing up the arp during the beat. Same thing for bar.(Would be nice if this could be chosen along with the above mode, so you could have 1-finger dynamic chord selection sampled on the beat or bar, for a walking bassline to input the structural chords.)
Similar to the above, but slightly different: in normal arp mode, a per-arp option to use a simple "best match" heuristic to choose the appropriate chord from the sampled notes, instead of using only the sampled notes.