Hard to tell, depends so much on personal preferences. But in my case I'd like to see the following external buttons/potis:
- Arp on/off
- Hold on/off
- Tempo
- Gatelength (maybe not important for everyone)
If I had to build my controller from the scratch, then I would add the following:
- Range switch
- Mode switch/rotary
So basically the same stuff that's in my Juno6 which I use quite often. So simple, effective and you can play around with it in realtime.
Cheers.