Tone stack with bypass
effects loop
pre-volume
post-volume (master)
lo/hi gain inputs
4/8 ohm output : Might add a 16ohm for those Marshall cab folks!
If you wanted to get crazy you could make it two channel footswitchable: rhythm and lead.
Or add a "crunch" switch for the clean to make it easy to dial in distortion with your guitar volume control.
The only other thing that I would find useful is an abilty to lower the wattage for bedroom practice. That's the reason I got rid of my 5150 combo. Great, awesome sounding amp with a warm clean sound (way different than the heads) but it needed to be cranked to really shine.