Sounds great! Besides, needing to do ground and air tests in order to unclock eqipment is a great idea of it's own. I mean, think of all the testing! The aircraft parts could finally cpntribute to the tech tree (I always thought it's sad that you don't actually need them as all the science can be made with rockets and aircraft parts remain highly optional)
So what I see is actually contracts that allow you to permanently unlock certain parts
For example: a contract that makes you launch a rocket to a certain place and land it there will unlock better guiding surfaces (wings and stuff) as well as jet engines. Contracts that require to build and send to specific location aircraft will unlock additional parts. Occasionally contracts like "build a hovering plane" appear, with something like "hover for 10 seconds with horizontal/vertical speed lesser than 10 m/s", that will award you with landing legs and RCS system. "Drop a probe from orbit, we want to see what will happen" will reward you with heat shields, etc.