OK, analyzed my tremolo use. I use two wave forms. On my Keely I use just a light touch of harmonic trem to add just a bit of movement, and it seems to warm up the sound. Good for Dylan songs. On my Boss TR-2, I'll put the wave form knob right in the middle between the triangle and the square wave. I'll call that a center wave. Then I have to stoop over and adjust the speed and depth knobs as I ramp them up on a song. So since I'm just dreaming here ......
Somewhere on the face of the trem pedal there would be a mini-switch that would let me switch between harmonic and center wave.
There would be a depth and a rate knob. On the songs that I'd use the harmonic trem, that would be just fine.
When using the center wave, on the songs where there's a ramp up with the expression pedal, I'd set the rate and depth knobs at max, but after plugging in the expression pedal, like one of those moog pedals, both knobs would be controlled by the expression pedal, so at the beginning of the song there would be no trem and the expression pedal would be in the up position. As the song progressed I'd gently press the pedal lower after each verse, and by the third and final verse the pedal would be totally down, and it be total chop at a crazy high rate of speed. So the expression pedal would control both the rate and depth, and you could set your maximum depth and rate using the knobs. I find tap tempo too difficult but that might be my previous pedals, but the expression pedal works for me.
That's how I'd envision it. Crazy right? I hope that gives you some ideas. (Thanks for letting me dream, I need to think about things like that.) Using the Boss pedal there is no volume drop.