There’s already a CC for portamento rate, so I reckon were you playing this through, say, Ableton, you could make this happen through a device. But this plus a second MIDI CV pitch for paraphony direct on the device? That’d be amazing.
Yes, a slide mode which activate portamento when two notes overlap would be awesome. It would be perfect for that 303 stylish sound. And in the same way, a retrig/no-retrig mode would be really useful for expressiveness.