Artificial Intelligence User Interfaces (AIUIs)
UI has changed a lot over the years... or have it? The last major improvement perhaps was the touch screen. In fact, it was breaking enough that touch devices got their own operating systems (e.g., macOS vs iOS). But fundamentally it remains the same thing as mouse technology. So are Styli.
With the already existing abundance of AI, and with the decade of AI on the horizon, the next chapter in UI I believe will be a user interface built completely around intelligent input interpretation. Eventually, keyboards will become obsolete, as voice recognition, handwriting recognition, finger gesture recognition, will replace it. The user interface will adapt to your interactions with it, learning the patterns and gestures you prefer to do something. This is a complete reversal of the roles where currently you need to adapt to the user interface's patterns.
Additionally, apps will be replaced by their content. Instead of opening youtube to find a video, the user interface will recognize that you want or need to watch a video and it will display it in a unified space of results. The video will still be credited for youtube, but only as a service or provider, not an app. Same for, say, word processing apps: No controls, no windows, just documents and a UI smart enough to know how you want to edit or interact with.
The end result of all of this is a "digital assistant OS", that will extend current digital assistants so they are all you need on your device. This is intuitive UI taken to a whole new level, making traditional UI more and more obsolete.