No, you wouldn't be locked out of the arena for becoming a general, you'd simply retire that character to a Hall of Fame and start a new one. And again, I'm not saying this is how it will be, just throwing out some of the discussions we've had here on putting an end-game into it.
Missions come down from HQ (the host), players don't get to make missions. Rank and leadership roles will not revolve around what you're going to go do (e.g. we're bombing Berlin today) but rather how you're going to do the mission (e.g. Flight X is going to range ahead of the bomber stream, Flight Y is close cover, Flight Z is staying above, etc.) I was just tossing around the idea of perhaps allowing generals to do something more on the strategic side, but even if we did do something along those lines, it would limited and not a replacement of the HQ generated missions.