The Mark V is the pinnacle of Mark Designs. It's corrected any faults with previous Marks, and rolled the best of each era up into one extremely versatile amp. There's really nowhere to go now, unless the "VI" becomes it's own animal that just blows all other boogies away, or they come out with a IIC+ Reissue. :lol:
EDIT: I could see a Mark V rev B that fixes the BIGGEST complaint about the Mark V-- MIDI Switching. That's about the only thing that could be improved on.