I recently worked on a collab with the wonderful  TikTikKobold, a Sonic fanfic called Whispering Shadows. Here's his description:

"In an alternate universe, Project Shadow was a rousing success, and history for Earth and Mobius has changed in such ways that no one would recognize it. It is in this world that two beings who would be destined to have animosity for one another instead find themselves in a different situation: This is the story of Whisper and Shadow."

I got the idea to write something with a wolf-girl maid "servicing" her master from a picture of Whisper in a maid outfit I saw and Tik Tik helped me flesh it out into more of a proper story, introducing Shadow and the worldbuilding into this. In the end, we split the work on this very evenly, with him handling most of the Shadow parts and me doing all the Whisper parts. It was a great pleasure working with him on this, and I hope the reader will enjoy it as much as I did.