FlowGPT have been running game jams along these lines for a while now. I've been trying to put together a prompt that makes GPT act as a D&D *player* that a user can act as DM with—it's hard to wrangle but the sodding thing goes off in some wild directions, sometimes. It's fun, because it's almost entirely unpredictable, and a good DM should be resilient to unpredictability.
Here's an early attempt. Ignore the first wall of text—it's the prompt: https://chat.openai.com/share/3ac574f6-c49f-4b80-9e41-317487f69f2f