I know of xml mod, prop, and the new car class, but I still think it'd be cool to see more tournament... I dunno the right word.. but they are neat ideas I have. Some are probably hard or impossible to manage but still it'd be cool to see.
Ideas:
Super size : The plane must be at the maximum size limit (or at least close to it)
Realistic : the plane must LOOK realistic and such
Impossibility : it cannot look remotely realistic
Micro : instead of close to max size it must be close to a minimum size
Hybrid : the plane must have jets and propellers
Multi : the plane needs to have more than 1 engine
Single : plane must have 1 engine

That's about all I can think of. Again I understand some of these might not be easy to moderate but still it's a neat idea.
If you have an idea feel free to share it.

Ideas Plus (thought of them after posting) :
Speed + : the plane must be faster than a certain speed
Speed - : the plane must be under a certain speed
Activation : all activation groups are turned on
Pull up! : the thing I forgot the name of that moves the pitch control slightly, yeah that, it's all the way down / up