The only thing I've ever considered adding to the Dual Shock or most other controllers is an extra two face buttons, for 6 in total. I think this worked well on the Saturn and although it does break the simplicity of just four buttons in a diamond arrangement, I think 6 buttons is manageable, the Xbox did have 6 but I didn't like how offset they were, I'd prefer them to follow in line with the current 4 buttons. I don't expect many people would like it though and 4 buttons does suffice most of the time. Sony would never change their iconic 4 symbols anyway.
I don't much like the idea of adding more to already feature packed controllers which are ever rising in cost. So really I'd be fine with no additional inputs, it's a great controller as it is. In fact, I might like a slightly cheaper version without rumble.