I have have the Mustang III. The only thing that would be nice to have would be a simple all in one Mustang Floor footswitch to control the amp. The amp has two separate footswitches & another expression pedal to get all the switching features. I get by fine with just the 2 & 4 button footswitches, but an all in one package would be nice.
At 199 $, you can buy the Mustang Floor and use it connected to the effect return of the III, the inside of it is exactly the same as the Mustang III. Use the amp as poweramp, cabinet, and backup, and the Mustang as you would use a typical midi controller.
Almost every midi controller costs more than that.