It just occurred to me that the Switch 2’s MouseMode could be the only control method that the Steam Deck is “missing.” I would absolutely love to be able to play DOOM or Halo: MCC or Overkill from the couch. This would be the only thing I’d like more than the rumored Steam Controller 2 design.
For trackball, I personally find a finger controlled trackball much more intuitive and easy to use compared to a thumb controlled one. The trackpads on the deck and steam controller are great, but they generally are thumb only unless you have some really funky grip.