Would it be possible to create your own arp patterns, so they don't just play 8th or 16th notes. Something a bit like 'Figure' where you choose a rhythm and just hold down keys, but where you create your own rhythmic patterns?
I think this would work better than a piano roll. It would be like a customisable trance gate - you could have a row of 16 'lights' that you can create your rhythm on by tapping to turn a light on (this could be a pop up window so it doesn't have to be squashed onto the interface)
Hope this makes sense