Have you seen NovelAI? That's kinda their whole thing is a copilot when writing stories. I don't know the tech they use but it could just be GPT-3 in the backend.
They use a combination of GPT-neo models from Eleuther and Facebook's Fairseq models -- the special sauce for NAI is their fine-tunes on those models though. They have a free trial so I'd recommend checking it out if anyone wants to play with it.
