This is rather neat, I might actually play Zone if there was more to do than just run arround and shoot people!
Perhaps you have some sort of "level" associated with how many missions you complete and when you go to the next "level" you get access to more things?
Anyways I was also thinking of something else...
Bonuses (money, "level exp," whatever) for achieving certain things.
Like never be seen or find a certain number of items or what not.