I’ve been fascinated by the idea of attaching multiple mice to one computer, and then having multiple mouse cursors inside of one desktop environment!
I just spent three weeks investigating how well that’s currently supported on Linux & Wayland. Let me tell you what I found! The results are surprisingly cool.
↫ blinry
It shouldn’t come as a surprise, though, that a lot of graphical user interfaces don’t really have any affordances for multiple mouse pointers manipulating the same object (e.g. one cursor drags the window, another tries to close it), but the possibilities are really endless here. It’s not quite there yet to be considered fully-featured and plug-and-play, but it’s a lot more capable than I thought it would be.
The first thing that came to my mind is sitting down behind my computer with my kids to teach them the basics of using a GUI. Instead of having to pass the mouse back and forth, one of my kids (or both!) could have his own cursor, which is a lot more fun and collaborative. Games are obviously another great application for this sort of thing, especially things like classic board games.
I hope this gets some more attention.

As the author also found, my experience from a long time ago was that this is a surprisingly effective way to use the desktop (warning: some personal nostalgia follows).
Back in 1996 I wrote a module for RISC OS that allowed two mice pointers to be used simultaneously (one controlled using the actual mouse, the other using the keyboard). Even with a much more constrained system (e.g. RISC OS rendered the mouse pointer in hardware) it was a lot of fun and generally worked well. The main caveat was that full control had to be given to a single pointer during a drag, so it’s nice to see this restriction (largely) no longer applies.
https://www.flypig.co.uk/riscos#:~:text=twomice
Collaborative two-player Lemmings with it was especially joyful (and made some of the levels much easier to complete!).