Is there or can there be a system like that of CW3's Shield Keys in custom maps? Basically this amounts to a method for a mission to read whether a different mission has been completed. In the past this has been used to create some fascinating concepts, such as J's Unit Map Series, where each map gave you access to a new unit, and every map knew which units you had collected thus far.
That would be perfect
This concept was addressed in a different topic already. I'll post it here for anyone reading this topic but not that one.
http://knucklecracker.com/forums/index.php?topic=21601.0