I thought I posted this a couple of years ago and I also thought it was submitted on the official list at AU but, maybe I dreamed that..
Anyway, I'd like to see where you can have a session of Autocad, Autocad MEP, or Autocad Arch open, then open just a viewport on your second monitor. No more stretching one session across the monitors and trying to line them up so the split is right in the middle. You'd be able to switch back and forth between viewports but, your tools and palettes would stay where they're normally set up to. Hmm, maybe put them on a 3rd monitor..
Anyway, I'd definitely like to be able to make better use of my large dual monitor set up but, to have to go back and forth across the monitors is a giant pain. The main monitor would have the whole ribbon set up on there. The second monitor would be primarily a workspace.
With users working with quick keys and shortcuts, I don't think that would be any kind of productivity problem at all.
I guess I'd rather see 2 viewports on one screen and one on the other, zoomed to the full size of that screen instead of 1/2 of one of my monitors or spread over two monitors and wearing out my mouse going back and forth to hit the ribbon..