My dream is get the PPI somewhere around 5k at 27/28", but use all that bandwidth for an ultrawide display, so you don't need it to be so tall and don't have the issues you get with 2x displays, such as a bevel in the middle and no real ideal layout.
Responding from my Dell u3417w, I have to agree that ultrawide is a great form factor for a monitor. The PPI of my monitor is about that of an Apple Thunderbolt display, so I do miss the "retina" experience of my Macbook Pro driving the monitor, but overall having so much horizontal real estate is glorious. I think my dream, too, would be a monitor with these dimensions but a higher PPI.
If I could have beaten my Mac into driving the LG at a non-2x scale, I'd have kept with it, it's such a nice panel. But now it's on my Win 10 box, which supports it perfectly; that was a very pleasant surprise.
I am not happy about its brightness level -- it's noticeably dimmer than an Apple Cinema Display, and turning brightness/contrast up just gives you a washed-out image.
The other thing is that the width, for me, doesn't add much utility. I like my windows centered so I am looking straight ahead at what I am working on, and keeping certain things (like log outputs and dashboards) in the periphery of my vision is just distracting. I know some people enjoy tiled window arrangements and vertically-split editor windows, but that stuff just doesn't do anything for me. So I am not using the horizontal space very efficiently. The best use of the width is being able to read wide diffs or other things that go side by side.
I also couldn't get it to work as a USB hub, not sure why. Thunderbolt would have been better, at any rate -- then you'd only need two cables instead of three.
I am considering replacing my home monitor, a Cinema Display, with the Dell 27-inch 4K, but it's just so damn ugly. It's got the thick bezel instead of the "infinity edge" one or the black glass of the 5K (which I can't buy since it requires two DisplayPort connections).