A multiplayer campaign would be fantastic.
It would be great to have a series of missions that took place in a similar geography, with similar types of threats and resources available to the players. After a few missions people who flew together would have a sense of the landscape and where good targets were likely to be found, and where the well-defended targets and threats were.
It might feel more like an air-war than the series of unrelated missions we have now.
As for number of players...... I usually fly as a two ship, and a 4 ship at most.... but that is because I only fly with people that I really trust; people who take the time to plan, brief, and debrief each mission.
Keep us posted,
Chris