I too love trackball mice, and had to start using them for gaming as my hands / wrist could no longer do regular mice for long periods of time. I've only tried a couple of the Logitech ones. First the Trackman Marble, but that one would make the middle of the back of my hand sore. Second, I tried the M570, and that has worked surprising well. I've reprogrammed the extra up/down buttons to be double click and toggle holding right click respectively, and that has worked well for me and a variety of games. Which trackballs have your tried for gaming? And what are your goals for the one your are creating?
On my desk currently are the Kensington Expert Mouse and a Logitech M570. I'm rather unimpressed with both for multiple reasons, and through design iteration have found that my design most closely resembles the discontinued Microsoft Trackball Explorer, although mine has a larger ball and a different scrollwheel, as well as a different switch mechanism for the two pairs of buttons. My goal is to make one that allows you access to all the buttons without moving your hand, which is a major problem with the Expert Mouse, as well as giving the precision of a large trackball-- which I feel is easiest to achieve with an index-finger controlled trackball. I also want to make it ergonomic, since the Expert Mouse seriously lacks in that front and I still get wrist strain using it. I think it's a fairly sizeable untapped market and have a lot of faith in this design, assuming I can convince a manufacturer to make it. I'm pretty well along with my first generation of prototypes-- just waiting on Davis's 3d scanners to open up for the month so I can get my clay model digitized.