Games went from 2D to 3D. The desktop is still 2D. Here's an example of a login screen that is 3D. 3D isn't really useful on the desktop but cool.
We have even lost the pseudo-3D stuff and now everything is flat. That being said I actually think Windows 10 looks decent. MS has shaved windowborders so it looks cleaner than Windows 8.1.
Even the built-in Windows 10 startmenu is OK I think. What is missing from the standard Windows 10 UI are small tweaks. I would like background colour option for Modern apps. The Settings app is either white or black, both can be annoying to the eyes. Same goes for the all white Windows Explorer. I'm not really using Windows 7 anymore which means I lost the ability to use WB with start, taskbar, windowborder and Explorer textures on default MS theme. I used that for 7 years straight. I'm a little hesitant to real skins. I really appreciate the artistic talent and work that go into them, but compatibility can be an issue. I remember people complaining about Firefox. Firefox has no compatibility problems I thought, because the textures made me forget I was using the original MS theme.
I believe possibilities are endless, but after pseudo-3D (icons etc. which I liked), skeumorphic design (which I liked), things turned flat and we may have to wait for new tech and trends. I think real 3D is the future (like in games), but I have no idea if it makes sense on a desktop. I've been using Google Earth recently and it has a 3D view that is both cool and useful.
I spend most of my time in a browser anyway so if the future is going to be cool or not also depends on the kind of content that the web delivers. Latest version of Google Earth is a web-app for Google Chrome so pretty cool stuff can happen in a browser. Maybe 3D websites will be a new thing?