I somewhat agree with ftx, in that this new gamemode could introduce some very weird, probably unfun strategies in games with experienced players. I'm not sure that really matters though, as most experienced players will stay with the current ranked modes anyway.
Your suggested mode would be a great tool for new player onboarding though!
The coop campaign works on the "loose only if all ACUs die" rule already, and I always thought it made teaching FAF to complete newbies a lot easier.
I wouldn't put a lot of time into developing the mode, the target audience is probably rather small, but if you can cobble it together without much effort it sounds like a great idea