Before you continue reading, I just want to say I know this would be very complex to implement, and my idea of it probably has many details that have not been thought out.
Lately I've been noticing a lot of people suggesting a lot of good ideas for missions. I saw an idea on the Facebook page, and thought it was a good idea. It would be interesting to have so once you've earned a X amount of credits, one could create their own missions.
Further more, it would be interesting to have something like 'mission packs', like graphics packs except for, for missions. You would be able to use multiple of these, and naturally they would have to submitted, then approved by moderators. Or for an alternative, maybe have a overall database players could contribute to, then the individual player could choose which missions he/she would like to use.