A strange mare (or stallion?) arrives in Ponyville, with even stranger tales of who they are and where they come from. They claim to be a humble officeworker, but refuse to tell of where they work.
Even more puzzling, they appear to take on entirely different personas every so often, never once returning to any of their previous ones. With each "shift" brings an entirely different set of memories, identity, and more. They do not recall anything they have done in the past and identify by strange, non-equine names with every persona they take on.
Probably a hybrid slice-of-life/adventure story like what I'd normally write if I ever do write this.