HeDanny: Can't help but feel it would have been better with trackball.
Apple Pippin controller tried that. Anyway, I've been trying to use a thumb trackball (Logitech M570) with PC FPS games, and while I feel it is better than using an analog stick for aiming, I never got fully hang of it. I wouldn't be surprised that I'd find a "concave trackpad" like that on the Steam gamepad even superior to the M570.
Other potential things against a trackball:
- It is probably more expensive, breaks more easily (moving parts) and requires more maintenance (cleaning) than a trackpad.
- The trackball could probably emulate only a mouse. I think a concave trackpad can emulate both a mouse, and an analog stick. So, if there was a separate trackball on the gamepad, it should also have the separate analog thumbsticks on top of it. It could become quite crowded design... I think the old Apple Pippin "trackball gamepad" didn't have any analog sticks at all, so it was basically just an old SNES kind of digital gamepad, with a trackball.
Think how you would play e.g. a racing game or a space combat sim with the trackball.