From my point of view, the first and foremost immersion killer is the "bloodthirsty animal" behavior of the AI. I am not talking about aggressiveness, but persistence of the interception. The reason why every dogfight eventually becomes a bloodbath, is because the AI won't settle until it sees it's enemy hit the ground. If a dogfight occurs at 10.000 feet above the lines, and one or two aircraft decide to run for it, diving towards their side, it is almost sure that someone will pursuit them. And the fun part is that the guys chasing them will not break the chase even if it is 10 miles behind enemy lines with flak all around. In WWI, if someone decided to dive towards their side of the lines, it would be almost sure that he would not have a madman on its tail, ignoring every danger just to get the kill. What I want to see is a more "human" AI. MUCH more defensive and careful. Not pansy AI though. If we turn round and round and he gets on my six, I want him to chop my tail to pieces. (and in P3 this happens just fine) I want tacticians, not enraged barbarians who will chase you up to Berlin or Paris. And of course, aircraft which have been damaged, must fly towards their side more often. But since each aircraft is piloted by a different person, it would be a cool feature if each pilot/aircraft had a different "stance" which would be determined let's say by a die roll at the beginning of each combat. One pilot may be just aggressive but careful, another would be a pure chicken, another would be an absolute madman etc. etc. If this stuff could be implemented, then the casualties from both sides would be significantly lowered and maybe you wouldn't get 3 kills per flight as a player.
Another nice feature would be this: When the AI is on the lead, and you are under attack by flak, it would be a great idea to split the formation and proceed to evasive flak manoueveres. I am talking about the AI pilots of course.
Random engine failures.
Ability to fly alongside Aces in the same sortie.
Ability to fly on a lone wolf patrol once in a while, whenever you decide, if you are an Ace. (just like Bishop did)
And of course, one very important feature would be to connect the mission with the campaign, when it comes to squadron casualties. When I see a wingman of mine crashing, it would be amazing if the OFF engine determines the outcome of the crash, and kills, injures, captures the pilot in the campaign as well. But this would be good only if the AI would be more defensive, therefore no more bloodbaths (or one bloodbath per 2 weeks or whatever...)
Just my two cents as a past P2 player, RB player, P3 reviewer etc. etc.