Whoops, I just heard someone talk about groovebox and I got too excited, haha. Looks like you were more talking about an actual synth instead of a complex sequencer here
Considering synths, I'm all about super small, portable, and battery powered for sure. 1/4" out if it's mono, too. For pots/sliders, obviously something to change the waveform and maybe a vibrato knob. Since it's in mono, a knob to control glide amount would be kinda rad. Buttons to switch the octave. If you throw in an arpeggiator, I'd love a tap tempo. Of course this means that the unit now needs to have some sense of time and BPM. And if there now is some sense of time, sync in/out would be amazing. If you aren't interested in doing a MIDI sync, consider using a CV Gate. The synth would just have to send out a short pulse blip on each quarter note for the Sync Out and accept a similar signal for the Sync In.