I'm going to throw a curve ball here, and propose a 100% compatible ( but still phone friendly) smartphone/tablet app.
Yes, my main DAW is a big PC, but I would love to be able to record stuff on my phone with reaper, and then be able to open the same project on my DAW and work on it for real.
the phone version doesn't need to have all the features, but.. you know.. enough to record something even if you're away from home, or even just don't feel like firing up the whole rig.
Because right now now, what I have to do is record something in a completely different app, export everything to .wav, and re-start a project in reaper.. not to mention MIDI is out of the question..
I know, recording on a phone isn't "professional" but these things are gtting better each year, and you can actually get really good results if you pair them with an audio interface.
Besides, I think that would open a whole new, untapped market for reaper as well
money money money!