I've heard of flight sims that are completely open domain but I've never heard of an online flight sim company that supports mods. i.e. by mods I'm saying that a guy can create a p38 or whatever with a graphics package, put in the correct parameters in it and have it submitted to the online game company for approval. If they like it they can put it online for everyone to fly.. if they don't the person can at least fly their plane offline and make changes so they will like it. Maybe if it's really good HT can have a "buy your plane" program, where you get a free month or two of flying if your plane is good enough.
This concept also applies to coding/creating additional elements including: infantry, tanks, jeeps and gliders. Although more difficult one could also develop new strategy systems coded in whatever language is needed and submit those for approval (e.g. plane attrition

).
What I'm trying to get at is this.. shooter games like quake, q2 and such really stunk when they came out. They only became fun only once 100's of members in the community started to contribute mods and code e.g. TF and AQ2. Now of course these are entire mods.. but I think an online sim would be even easier to mod. since all your creating is either new "vehicles" (i.e. skins/models in quake2) with specifications OR strategy subsystems. IF we could do this I'm sure within a month we would see MANY new planes being designed.... furthermore it would give someone that feeling they can fly something that they created..