I would settle for a 21.5 retina iMac, and a 21.5 retina display connected to that iMac, both side by side on my VESA desk mount.
That would be GORGEOUS. But of course it's not possible yet.
Definitely agree with you. A 4K 21.5" TBD would also be perfect for Mac mini users too. Remember when Apple used to sell three different sizes for their classic Cinema Displays? They really covered their Mac user base with display choices back then. I'm not saying we need to have that many now, but at least two seem pretty essential.