Like several others, my initial thought was a SpaceBlade Trackpad.
As another user posted - the unknown is whether it is possible to determine the intent of typing, Green chord, tracking, and clicking. It might be possible if:
Simple press on lower edge —> space or Green as currently done.
Surfade motion on the TrackBlade —> mouse.
... followed by no finger lift, then press —> click.
Multitouch for secondary-click and 2D scrolling.
... or Edit chord plus surface motion for 2D scrolling. Edit and Select chords plus TrackBlade Press for secondary and tertiary click.
Not sure we need more gestures because you can assign those on TextBlade.
Mash plus surface motion for drag (and Edit/Select chors for secondary/tertiary drag).
Or there could be two levels of click (like a camera’s half-press being focus, and a hard press being the shutter release). Might do more annoyance than good.
A wacky idea came to me: the optical non-ball trackball BlackBerry put on a few models many years back. They had a while glowing ball for most of their phones, but one model they replaced the ball with a flat white square that was (I’m purely guessing) an inverted optical mouse. I never owned a Blackberry so I cannot attest to how well it worked. But it could be made very small, unlike the Magic Trackpad with all its aluminum. Or you could embed one near the thumb resting spots on the SpaceBlade if coupled capacitance isn’t feasible.