Here's what I would really like (but nobody is going to build it): I'd like to upgrade the hard drive inside my MacBook, but with newer MacBooks that's unfortunately too difficult, so it's not going to happen. So what I want instead is an external drive that is connected to my MacBook, as permanently as possible. It will never be removed. I don't want it on the table. I want it in a safe place when I'm on the train, for example.
To do this: A very thin device. some material as my MacBook, that is glued permanently to the back of the screen. Edges smooth so it doesn't catch if I move it in and out of my bag. And a cable that is designed to go round the corner from the back of the screen to a USB port on the side of my Mac (not a straight cable that you have to bend, but one where the bend is part of the cable.
Result: You open your MacBook, and the hard drive is there. And since it doesn't have to look like a hard drive, it can be smaller and flatter. For extra goodness, a USB port at the back so you can attach two of them.