It would be such an amazing learning experience for any designer to create a UI/UX for TempleOS. Imagine being given the opportunity to design how OS X or Windows should look and feel from the ground up. Linux is obviously another option, but unlike with TempleOS, you would not be treading new ground, and it would be hard to not just evolve the Linux interface, rather than defining it from scratch.
It would certainly be a challenge given the display limitations of TempleOS the last the I checked it out. I believe the display was limited to Not that you cant create beautifully usable interfaces within such restrictive limitations, but minus all the spark and sizzle of modern UIs I think it would be a challenge to first get a UI designer _excited_ about creating a solution for TempleOS
The work of Susan Kare is held in high regard. I guess the pioneering aspect has gone and it'd be more like carving a pixel-art niche.