What would you want from a full frame Canon or Nikon mirrorless camera?

Nikon has finally confirmed the age-old rumor that its going to make a full frame mirrorless camera. It's widely assumed Canon plans to do the same. But what are the things they need to get right? We're way beyond the point where the 'mirrorless or DSLR' question simply depends on whether size or autofocus is more important to you. But what are the other aspects of camera design and behavior that need to be in place for a camera to work for your photography? We'd love to hear what you think, so ...