Ive tried to go through the list, but man what a list it is!
I like the game. Got hooked on it because of its simplicity, yet realism at the same time.
My opinions;
Graphically its good. Start trying to add more and it gets to where the lagging might become more of an issue.
I think adding to the realism, while keeping it simple, is the way to go for "improvement". Although its a good game in and of itself.
Adding more planes, of the WWI era, is probably the best approach I can think of and does not take away from the game as it is. I think adding 2 seat planes, such as the AEG C.IVN(although not technically a fighter, I think the Aviatik C.I looks better and would serve the same function of the AEG C.IVN) and Whitworth FK.8, would provide the 'cool' factor of having a true tail gunner. In fact, I believe both those planes carried more than one bomb so that would add the "bomber" type plane to the pack that people would like to see.
I also like the idea of the AA guns. It is a real thing that would serve a purpose and would not take away from the game at all.
Add in some night missions. Not sure if that has been said yet, but man that would be cool seeing the bullets whiz by you or seeing a plane going down on fire.
All in all, unless it is too complex, keeping teams even is the best thing to do. But along with that, anything that would help promote 'team' work would make it more realistic. I am sure there were a lot of renegade pilots out there during the war, but most of the time they worked together.
Oh, I like the choosing the plane color thing. It might not be period correct, but it would probably make some people like it better if they could choose their own planes color. I would personally stick with the colors on them already, but that's just me.
I'm sure whatever you come up with will be great, please just don't take away from the simplicity of how to play/fly. That is this games #1 advantage over other games I believe.
That's about all I got to say about that.