Santiago did the JVM, are you saying he's back at Marshall?
What I'd like to see Marshall do is take the OD1 channel of the JVM, tweak the green to be dead on 800 level gain, leave the Orange alone - that's high gain heaven IMO and leave OD1 Red alone, it's as modern high gain as you can get. I do think OD1 Red as it is now is a little underappreciated, I use it just for fun here or there but it's just got loads of gain. Then a Clean/Crunch that goes from pristine clean to plexi in the the three modes. And it has to nail the Plexi.
Then dual masters, remove the verb, add a gate, serial loop. If they kept the Serial/Parallel they have to fix the unity gain. With full mix there is just a slight volume drop. I'm fine with it as is but it does cause some problems.
Design wise, keep it a single row of knobs instead of the bigger look of the 205 and 210. The open space on those faceplates has always bugged me. As for the 800 and Plexi tone they have to nail that in the modes. You can get in the ball park now but you're just in the ball park.
Call it the JVM 100 and the JVM 50.
I'd also relaunch the JCM800 with added features, call it the 800SE. First goal is it has to nail the 800 out of the gate, that's a given. Then I would add dual masters, and Gain 1 and Gain 2. Gain 1 would be the traditional 800, Gain 2 would the modded high gain blazing 800 that everyone and their mama has done with the 800 and basically launced an entire industry itself. I mean afterall that is what everyone has always wanted from a stock Marshall. On the back I'd add 50/100 switch and I would take the attenuator from the YJM model and add that along with a serial loop and then whatever switches, channel switching thats needed to added. And if they can throw in midi, why not.
There done, new amps lol. Marshall, hire me!