As far as 6s offclassing, they could only allow offclassing if a certain number of people on your team agree. Seeing as it is actually Valve and not some third party they could easily implement something easy to use (1 key to let your teammate offclass, another to choose Pyro, Heavy, Sniper, or Spy). Also just lock the Pocket, Demo, and Medic to their classes.
They could also allow offclassing in ranked games of a certain skill level where people (presumably) need to be pretty decent to get to the level (like a mid IM equivalent) and just let them offclass whenever because if you are that rank you probably know what you are doing and aren't trolling.
Also voice communication in game is pretty laggy, probably not ideal for something like 6s.