Stock vessels contest.
A contest during alpha/beta stage for 2 vessels that will come into the game as stock ones. Users would make a simple, functional rocket that would show basics of game mechanics and a simple, functional plane doing the same thing. Devs would then choose which one is the best from each category and add it to the game, adding the user who made it to credits screen, too. And maybe some point reward for winners.
This isn't really a game idea, but I hope devs will take it into consideration.