I don’t think it’s so much a materials challenge as a manufacturing one. Putting a multitouch digitizer uniformly over the surface of a sphere is AFAIK a unique challenge.

I’m not sure how you could make the ball slightly squeezable yet hard and acceptably round to make for a nice trackball experience. As the OP points out, it’s easy to notice when things aren’t perfectly round. I think the best thing to do would be to just use the touch sensors, but I’m not super deep in the sensors world, maybe their is some sort of internal stress sensor that you could use to see if it is getting squeezed.

I think you’d have to switch the fixed magnet into the trackball, as I can’t see electromagnets of sufficient strength working on power beamed inside a ball. You need a “magic” algorithm and some fancy electormagnets in the base to try and attempt to create an “increased resistance” mode as you descri e. Alone this is a very interesting, and I suspect difficult problem that may have some prior art. I’m not an EE, but I feel like this is something that one might be able to do some back of the envelope calculations on to see if the adequate force is realistic given space and power constraints. One thought I had is that you might be able to use the resistance between emf fields and conductors, but without some sort of exotic metamaterial I don’t think you could get adequate resistance.

Anyway, I can only think of one company that could take this on, Apple. And if Apple got on it, one thing that would be neat is if the ball had xyz linear actuators so that it would click.

One of the coolest bits of manufacturing tech I've seen is laser-printed circuit traces on plastic. There are a couple different ways to do it, but in any case you can print gold traces onto 3D objects, even concave ones. It gets used to print antennas onto plastic for phones and whatnot. The tech is a little bit exotic but it's purchasable by even a small R&D group, don't have to be Apple to get it.

Do you have any recommendations for the metal trace printers? Or what keywords should I search for?

Rather complicated to do for the home lab, although I did come across someone's somewhat successful attempts to do so. The tech appears to be called Laser Direct Structuring.

Squeeze to click is such a cool idea for this. If it became a real product I could see myself buying one.

Logitech MX Master does something like that for its scroll wheel to go into click or freewheel mode: https://blog.bolt.io/logitech-mx-master-3-vs-2s/

I like the interaction ideas, and I think both 1 and 2 could work the way you describe, but 3 would probably be better solved inside the base. The bearing balls could be spring-loaded so when you press down, you engage with some other surface that acts like a brake?

Not super sure if that's what users would want, tough, it would probably make "throwing" the cursor harder.

The Logitech mx ergo has this with a little switch and it’s super useful.