I've been working on a single-player MUD concept on and off for years, mostly as an excuse to write.
I have a sweeping list of features in this thing at this point -- if very little meaningful content -- but my latest turnaround of this project has resulted in a fairly complex social simulation layer where, even though all areas, items, and quests are hand written, I use LLMs w/ tool calls to control the social simulation. For example, to get a quest off of an NPC you have to roleplay in-character and ask for it. Same for handing quests in, recruiting people, trading etc. Actions you take in the RP are tracked using a social log, and this can have personal consequences with the NPC, faction consequences, or even consequences within their immediate and extended social circle.
I have spent a lot of time messing around with apps like Aventura, SillyTavern, and now Marinara Engine -- but I don't think I've ever played a proper RPG that uses LLMs to bring a pre-existing, hand-defined world to life.
I'm sure somebody must have done this! Anyone got links to projects like it?