had a brainstrike(like a lightning strike) about a different way to unlock story arcs
i know this might be a bad idea, but have them unlock after doing a particular part of either: patrol, defense, or exploration missions. was thinking it might add to the background about why you are doing an arc in particular. basically, during a routine mission, you come across something that makes it a bigger mission than was originally thought, making it seem like a random encounter.
this would also give a reason to do those missions some people think are worthless.