I still think that a union of 
@WizardStan's thumb trackball mouse and a 'normal' mouse would be very, very cool.
Or maybe one with a Pyra nub instead of the trackball.
Or maybe one with a steam controller type touchpad instead of the trackball.
Somewhere I have an old pre-USB joystick port gaming control system that has a 4-way hat at the thumb position.
If Razer can put a 12 button numpad on the side of a mouse, why not have an analog device there instead?
I think this forum's membership could design the 'ultimate handheld controller mouse device'.  Maybe have it be modular so that the right thumb controller could be swapped between a pyra nub, gamepad cross, 6 button set or track ball.  Make a right and a left handed version that take the same modules.  Allow the modules to replace the left & right buttons...?
The trackball in WizardStan's controller is probably about 20mm.  That is the same width as the MOUSE1 and MOUSE2 on my generic desktop mouse.
So, a slightly larger mouse body that will take modules for the thumb and left/right mouse buttons.  Any module can be used in any of the 3 positions.  Left & right handed versions take the same modules.
The mouse internals would be a laser mouse and 4 port USB hub.  Each module would be it's own USB device.  Three devices + laser mouse plugged into the internal hub = 4 port USB 2.0 hub.  Computer sees 4 separate HID devices when it is plugged in.  Connects to the computer with a single Type C device cable (wired) or bluetooth 3 (wireless) - pick which one via a recessed switch on the bottom.  Two AAA or AA batteries for wireless use.
Modules:
trackball
nub
single Cherry MX brown switch
gamepad cross
gamepad 4 or maybe 6 action buttons
Type A USB port.  Plug in a thumb drive and boot from your mouse.
Steam Controller touchpad
microphone & speaker (instant speakerphone)
Blank - to cover the hole if someone doesn't want a module in that position
Thoughts - should there be dedicated left & right handed versions that each take 3 modules or make a single symmetrical 4 module version?