I know exactly what you're talking about. There used to be chat rooms like that. The "GM" would start the room and describe the setting while playing the pub-landlord/innkeeper. And people would just come in and roleplay. They would describe their character's physical appearance in first thing and then jump right into interacting with whoever was there. It was full of improvisation and great fun. But I don't know if you could pull something like that off in a forum.