Mission/Quest Style Gameplay

Idea for utilizing a Missions & Jobs style of game play. (think Freelancer)

My thought on this is two-fold.

#1 - Game will have an end.
. . . but
#2 - Endless repeatability.

So the idea is that, you have two types of levels you can choose to play.
Missions - These are the levels that are completely static. Static Bosses, enemies, everything! Missions also move the story forward.
Jobs - These are randomized levels, with scaled difficulties. Some have bosses, some don't. These levels can be used for; fun, gain experience, money reputation or whatever else we decided to throw into the game.

Missions don't necessarily come 1 - 1, you may not have met all requirements to do a mission. Missions may be optional. etc..
Jobs always available, and that sort of thing.