The big incoming UI challenge is XR. But as with ST/Alto, it's an integrated bootstrap problem of hardware, system design, foundational software, user software,
and UI.
Absent research labs, we wait on market availability for hardware. For the modern analogues of mice and bitmapped screens. Eye tracking may be cheap next year, but now it's $2k. High-resolution HMDs may be $10k next year, but now they're simply unavailable. Hand tracking is flaky. Haptics are "it buzzes".
For software, we're so used to the 2D windows interface, it's easy to forget how much foundational work was needed. Eg, around topography, and Smalltalk, and live coding. Hand gesture recognition, spoken dialog management, 3D constraint-based layout, are just a few modern analogues. And so on.
So it looks like we're going to be doing broad-based innovation and infrastructure construction, gated on hardware availability. And plagued by patents.
It's not clear to me what can usefully be done to explore UI in the meantime. You can do wireframing in TiltBrush, but not really. It doesn't give you the feel of say hand-attached controls, and of automated collaboration in general. The "oh, it feels good to present this graph as a sphere, and hold it you lap and peel off layers". So...
Perhaps do UI design by paper improv? A crew scribbling on and holding up paper, hovering around a "user", who is managing a todo list? :) That could be fun.