We can certainly do that as a community, the Untold Stories mod is great (find it here) 
Having said that, it’s unlikely to ever be pushed by FC rather than the community. Player created content is often a mixed bag when it comes to quality, and sadly the great stuff is all too often the minority. The official missions are all about quality, and being FC approved effectively puts a stamp on stuff saying “this is good”.
If they can’t be sure that the same quality is being delivered, then it’s a risky move from the developer position to do anything which could be seen as them endorsing low quality content.
Even if 99% of the stuff is great, they probably wouldn’t go much further than saying “this is some really nice player-created content”, because of the potential for it to come back to bite them.
There may be other reasons too - say someone created a mission where Deputy Andy has been using cats around KM to help draw Cassie’s eye to him. Not necessarily anything too bad, but it long term wouldn’t mesh well with the Meowling and Andy’s current existing romantic interests. If there’s another mission about Andy getting set up for his first date with Moose, then it starts getting more complicated and confused about how all the missions interact to create a coherent whole.
Looking at how many different angles and perspectives stuff like fanfiction generates, it presents a fair bit of difficulty for that to then be officially endorsed.