My suggestion is not to try to compete head-on with the big guys, but develop a feature (or features) they don't do well, then if it catches on, they will buy you out.
I still think the main undeveloped feature is an aid to finding partners and clubs. The site could offer some sort of pre-qualification for those seeking a plane to assume part of, or it could offer an evaluation or unbiased review of the plane that is seeking a partner.
Another feature could be a database of options. When I read that the plane I was buying had a "magic hand" and a "Brittain" autopilot, I had no idea what they were or how old the were. I would have liked to have been able to clicked on the item and read a review (eg: Brittain is one of the original full featured APs, but it is of an older analog design as opposed to the newer digital designs. Support is still available but some parts may be difficult to find).