Retina is not a fixed DPI. It is a combination of distance and detail. Your eye can see a maximum of like 3840x2400, and it does not matter if the screen with it is 24, 27, 30, 40 or 60 inch. The old IBM 22 inch Retina displays from 2005 had this resolution. DP can handle that, no problem, video card too. You can skip many anti-aliasing routines, which already downsample like 3840x2400 to 1920x1200, so it is not much more work.
Problem is, OSX was written from scratch to support fluent scaling, but since Apple never used its own potential, others did neither and this feature in OSX never took off.
But coming back to your question: there was Retina at 22 inch in history, but it did not succeed. It will return, be patient. Try Ebay for an IBM T220 or T221. Looks way better than TB displays too BTW!
Oh, and we want 30 bit colour too!
I seriously wonder what OS is going to support it. OS-es seem to drop professional support and move to consumer products. Windows had better high resolution high color support, but it took the fancy road with Windows 8 to follow iOSX.
For the time being: give your eyes some relieve and stick to 1920x1200 @ 24 inch or 2560x1600 @ 30 inch (hell that is a nice screen to work on!). The new 2540x1440@27 inch is way too tiny to my taste.