What if modest rear brake pedal (0-50%) controlled regen progressively, enough to cover 95% of all speed reduction, while firm rear brake pedal (50-100%) engaged the hydraulics? It might be intuitive, without any new controls, while maximizing regen and obviating the need for ever replacing pads or rotors. So while going down a long hill, we'd just drag the rear brake like normal, except we wouldn't be wearing out brakes.
Controlling regen, aka rear braking, with the front brake is only marginally acceptable to begin with. It's not acceptable for off-road applications. For on-road, it's one of those things that takes away control from the expert rider on behalf of the average rider. Therefore it's something that needs to be enabled or disabled per rider. Rear brake regen would not require such settings.
Rotating the throttle backward could be reserved for reversing from 0 to -1 km/hr. That would also be awesome, and iintuitive because throttle controls speed. We'd get another big feature that comparable ICE bikes won't ever get.
One other idea: if regen is strong enough to slip the rear wheel, and I suspect it truly is, and so it could handle 95% of all braking, maybe rear ABS could be implemented by pulsing regen and save some hardware.