Hmm. Honestly, this is really the opposite of the way I'm thinking. Here's my question: why would I be interested in making a device for "average users", tailored for content consumption? Apple and Google have already perfected that device.

You say Linux does not have a successful desktop environment. That it doesn't have a good programming IDE. On the other hand, I have to say that I've been using various Linux desktops for, well, two decades now. And that I do the majority of my coding in a shell in Vim. I honestly don't understand why I would want to use an iOS or Android like GUI, or a graphical IDE. I've been told over and over again that those environments are better for users; but all I see is that they are better for novice users. Once you get up to speed, the workflow actually becomes impeded by the amount of hand-holding provided by the environment.

Rather than create a UI tailored for the "average" user, I'd kinda prefer a UI that just tries to stay out of the way as much as possible. Maybe something like this: by default, dedicate the entire screen of the device to the app that currently has focus. All single-finger gestures, and pinch/zoom gestures, are routed directly to this app. For system commands, two-finger gestures are used: a two-finger swipe up from the lower half of the screen brings up a "command center", with the on-screen keyboard, settings, and anything else absolutely necessary to managing the device. Two-finger swipe back down to get rid of it. A two-finger swipe down from the top half provides access to the menu bar of the running app. A two-finger swipe left or right allows you to switch between running apps.

There would be no built-in e-mail service: that would be provided by an app. No built-in browser -- also provided by an app. No built-in app launcher! That would also be an app.

All that the GUI provides is the absolute minimal amount to manage running apps, and otherwise gets out of the way. Essentially, the display is entirely handed over to the current app, with the bare minimum of extraneous UI elements provided as an "overlay" on top of that app...