Without any aircraft, there is no flight simulator. Aircraft development is one of the fundamental parts of a flight simulator. Contributing is fun and can be done at all levels of experience.

All the way back in May 2011, we adopted a new status-rating system for aircraft. So far, only a few have actually been rated, as can be seen in the list 'hockenberry' set up at Google Docs. If you're an aircraft developer and your aircraft is not on the list, please consider rating their status. All you'll need to know/do is described at Formalizing Aircraft Status. If you'd just like to get started contributing to FlightGear, this would also seem like an excellent way to get started.