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.