I spend a great deal of time and energy writing in Shared Worlds. The concept of a shared world is thus: You are adding to a universe that other writers also add to and may use your characters/plots/materials from and move them in directions different than your original intent. In this way plots and story and characters grow in unexpected ways. There is many positives to be found in this sort of writing–namely you’re building on a world that is pre-existing and you are pulling on complex threads and knitting them back together in ways that others may not have intended or others may do the same in ways you never dreamed of. All of this is the knife’s edge of good and bad really. If the person who picks up your thread does terrible things with it, then you may wind up stuck in a situation you don’t like or it may alter a character/scenario you’ve had longstanding plans for in a way that cancels out your plans.
I write for Shadowrun. There are about two dozen writers moving through these books on a regular basis. Often times I step completely away from a book when I feel I have no story to tell (i.e. no skin in the game). However, in doing so I watch the world advance in ways I could not have predicted and it makes my ‘thematic goals’ harder to achieve. All of this is to say, writing in shared worlds means letting go of preconceived notions of character and plot. You have to be able to adjust to these changes on the fly and build what feels like the natural next step to whatever changes come your way. I’ve been at it for a few decades now and I can tell you it never gets easier, because you never know what is going to happen next. I can also say it has helped me shape plot and story spectacularly, because I have to learn how to adjust and uncover what feels natural. Overall, it is worth the doing, if only a few times.