Formation flying is a skill, not a feature. Formation flying must be achieved through PRACTICE, not by a push of a button.
Ofcourse we do have auto-trim settings which loosely corresponds to 'autopiloting'. Auto level trim(X), auto speed trim(alt+X), and lock pitch trim (shift+X). However, this is to reduce thr workload of pilots as a game play concession.
Admittably, the same logic CAN be used for formation flying too. However, if somebody wishes to fly at formation, in a sense he is already giving up the comfort of flying at whatever condition he likes.
Unlike the real world, AH doesn't really have a need for formation flying, since icons are a dead give-away for spotting planes out from afar. Formation flying in the real world was mandatory in order to maximize the efficiency of each of the pilots scanning the skies for enemy aircraft. Thus, formation flying in AH is more of a show-off and a testament to the skill level of each of the pilots in the wing. Instead of flying whatever way he likes, they will purposely give up the comfort of individual flying and form up a wing.
Besides, as long as the squadron is organized, the flight leader can always give out a heading, set throttle setting, and notify his wingmen of auto trim settings.. the leader will say something like "0-9-0 degrees bearing, 45" manifold". Then the wingmen gain up on him, and then set their planes to the same settings. And then, upon the leader's command they all engage alt+X and they will be able to fly/climb in formation easily. Any slight discrepancies in flight can be altered, and then reset.
I don't think there's any need for a 'formation flying' button. Since you can already do it easily enough - use the basic system, and mix some skill into it. That's good enough for gameplay.
Take away the skill level factor and just making it a single button feature is gonna take the fun away also.
