The ideas I had were to design planes/zeppelins for specific roles and missions, and maybe not to limit to just one flyer either.
For example, Nation A needs a cargo plane and escorts to ship a valuable cargo. You have $20,000 to design and build the cargo plane and 2 or more escorts.
Or you are the leader of a starting pirate band who has eyes on a delicious passenger zeppelin with all it's rich occupants. Design a band consisting of at least 3 planes with at least one being a cargo plane able to hold 10 boarders. You have intel suggesting that the passenger liner will have onboard security and probably 2 or more escort planes. You have $20,000.
Or maybe have two angles, one competition for the pirate band, and then one competition for the passenger zeppelin and escorts and then with all voting secret see which one wins and then hey, someone could then actually play the game using both wining designs and see what would actually happen!
You could also factor other things like range, minimum speed for racers and the like. The sky's the limit!