I think a day add "totally" a new plane was possible, but i think the devellopers need time to reflect how to do that correctly for the community, new tools, etc etc...
And remember what Matt have said during the SimHQ interview about FC2:
Q: Will more aircraft be able to be added by 3rd party modders, or will you have to swap a stock plane for a modded plane?
A: For now, third parties can still create and swap in new units. However, we are looking into providing "blank slots" for later projects.