Okay, based on Mike's post, I think maybe I'm missing something, so I decided to illustrate what I meant. (JJ2+ only.) The log on the left demonstrates how to have individual missions of more than one level in length; the log on the right shows multiple missions in a single planet where you have to go to the next one by choice. The two systems are obviously and easily combinable and you can play all the missions in any order you like. What else is necessary?