I can probably jump on this one.....
For starters, I would base the next gen Mark series off of the JP platform. It can still be in the medium class head as it is now.
1. power supply from JP, if this is what makes it so tight and responsive (based off the IIC+ coliseum 100W head).
2. Midi control that also replaces the simple logic controller for the footswitch. The footswitch on the JP is tied into the midi function. It works flawlessly. Mark V footswitch tends to misbehave sometimes, mostly cable related but have had more issues with it using variac power mode.
3. Dual 5band EQ is much better than the single with presets.
4. Keep the multi watt settings, and the full or variac power.
5. There is a lot already going on in the Mark V, what could really be next?
6. Instrument level friendly FX Loop is a must. The line level FXLoop on current V is not compatible with most effects.
7. switchable power amp, if possible, Simul-Class extended Class A push-pull that can be changed to full Class A/B. Call it Multi-Class
At least #2, #3, and #6 would be better than nothing.