i'm not sure more buttons are needed but maybe more inputs. the rumored touch screen input on the cafe is a good idea. i wouldn't mind if sony/ms ripped that one off right away. has to be multi-touch though.
analog face buttons perhaps, more than just the pressure sentitive buttons on the dualshock.
better accuracy motion detection.
two-handed controler for better freedom of motion.
the ir-pointer worked into the "standard" controller instead of a dualshock/move distinction.
incorporated body (kinect) controls with regular controller.
voice control.
i dunno what else -- i'm not a creative. but in general, more, better, and diverse set of inputs is a good thing.