Model it after a Smash TV panel. Put 6 buttons between each set of two sticks. Use a Happ Mechanical Rotary for each player's right hand stick, and a Happ Super, Competition or P360 for the left hands. If you decide to go for a spinner, put it somewhere above the sticks/buttons. There's a small footprint spinner available that can fit just about anywhere. (Slikstik Tornado?)
Then, either make it a wide panel and put the trackball in the middle, or make it a deep panel and put the trackball above the sticks.
Or do swapable panels or a modular system.