Instead of how the missions are currently spread out every 20 minutes, when a player enters the arena he can select from a list of missions that are starting at the same time. If he wants to fly an interception mission, or an escort missions, etc., he can without having to wait up to a possible 80 minutes for a mission he wants to fly.
Concurrent missions would be desirable, however like HiTech said, you could just have different spawn times and different flight paths to simulate varying flight objectives. One could also have different arenas, for objectives for example: Pacific arena for carrier to carrier action, BOB arena for bomber, escort and intercept missions, etc.
Although I very much appreciate your idea, the single most limited area of missions seems to be the lack of tools to make GV missions. Even if all the AI ground vehicles did was fire, move a short distance back, fire, and then move a short distance forward and fire again, with "calculated" accuracy (say, like 10% of their rounds fired are fine tuned to a calculated "hit" position). All other rounds fired by AI would be "ranging" (alternating, fore and aft at varying 0 -359 degrees of calculated target "hit" position). AI vehicles on both sides should eventually succumb to damage, and a player controlled vehicle would have more options for cover and accuracy.