I would like to see some sort of EQ/dynamics processor. Something with EQ effects like the FA-1, SP-1, PQ-4. Add to that some wah and envelope filter effects with an expression pedal connection for the wah. Finish it off with a selection of compressors.
I would like to see the rare Boss PN-2 Tremolo Pan & Boss TR-2 Tremolo & very rare Boss VB-2 Vibrato reissued in a Boss Twin pedal and take it to a higher level. Stereo inputs & outputs for deeper/wider effect for guitar/bass/keyboards etc... Control with optional expression pedal and/or twin pedals of rate, depth, square/triangle/sine waveforms, square/triangle/sine stereo panning effect, rise time, latch/bypass/unlatch modes, output level to compensate volume loss when the effect is engaged. Call it the Boss VT-20 VibraTrem. I would certainly by it!!!
PS-20 combining the best of the PS-3 and PS-5, plus arpeggiated pitchshifting.
An RV-20 with room/hall/arena/plate/spring/gate/reverse reverb and echo in true stereo... any of which can have a modulation applied to the reverb. And an 'octo' mode like the Verbzilla. Also a hold mode for infinite reverb (as long as the pedal is depressed).