ReTouch Control. I have it, it works well though is a bit complicated. For just mixer control, there's also rsTouch which is also quite excellent and much easier to set up, though does less.
Personally I think the controller space is pretty full already, especially given how many seemingly fundamental features many of us feel Reason is (and always has been) lacking. I won't go into what they are as I'm sure most everyone around here is all too aware of them. ;)
I'd really more than anything, like to see a purely feature-driven update. And by feature, I don't mean new synths or other devices, I mean all under-the-hood improvements to the sequencer, audio editing, MIDI functionality, etc. etc.