This list is a good example the answer to "Why doesn't TK listen to the community?"
This thread started out with a few of the very commonly requested features (IMHO) from over the years. Improved A/G avionics, working FAC, and mulitplayer for example. But then as soon as we start to put a list together, we get things that, in my 6 years at combatace, I have never even seen mentioned, let alone requested. Working sonar? Submarines that can run underwater? Seriously? And then there's some things that, while they make more sense, start to fall outside of the bucket of "big bang for your buck" stuff (neutral sides in campaigns, better radio menu, ability to specify specific aircraft for parking spaces, etc...). I agree those things would be nice, but they wouldn't get me playing the game again. And I certainly wouldn't pay for them. Again, just my opinion.
I like swabast's idea about developer kits. I doubt I'd be able to use one, but depending on their capabilities, I could see it being a major game changer.
My suggestion would be to let this thread run it's course for a few weeks/months to gather ideas, and then start another thread with a poll ("What features would you pay to have TK implement into the game or release as DLC") to narrow it down (maybe needs to be done multiple times) until we have 2 or 3 things. Then someone with the right personality, communication skills, and some understanding of the business could work out best way to propose it to TK and handle the communication.
My 2 cents.
edit - I see Stratos has already started to consolidate the list and proposed a poll.