Come on Apple, get you collective ass back in the computer industry. Compared to the cost of some resent acquisitions is should be relatively cheap and simple to offer three displays plus an ungraded mini.
Have a 21.5" HD display based on the current iMac 21.5" model. At a reasonable price a lot of Mac enthusiasts would opt for that size. Even Mac Pro users working in HD video might want one for media playback.
Next offer a 27" display based on the current iMac 27" model. 27 inches is plenty enough for most people and would work well on MBPs, rMBPs, Airs and minis.
Finally sell a 4K monitor for the big boys and girls either working on 4K or just wanting to spend the cash on the top-of-line ACD.
Give all monitors three or four powered USB 3 ports, two TB 2 ports, an Ethernet port and a FW800 port for backward compatibility. $599, $799 and $999. Really Apple, get your act back together.