People have been putting Blackberry trackballs on QMK builds for quite a while. I'm odd - what I want is a stupidly big trackball, like 4 inches across or so. And it should be able to detect rotation about the vertical axis. It infuriates me how optical tracking systems are designed to provide just translation and no rotation when there's a whole other DoF in play.
6DOF devices have been done (ie https://en.wikipedia.org/wiki/SpaceOrb_360 or https://www.computinghistory.org.uk/det/12416/Logitech-Cyber...). They're sorta fun to use (at first) in CAD for browsing 3D models, but never caught on as once you get good at the CAD UIs, the extra degrees of freedom get in the way more than they help.