There is a game i dream of, wich is pretty much impossible even for a full AAA team. I just use that to try to build parts of it to learn stuff.
The idea its a fully procedural RPG. and when im talking fully procedural, im talking 100% procedural. The stories, the land, the monsters, the quests, are all randomly generated. Like in minecraft, you can just walk in a direction and find new cities with their own stuff and monsters to slay. World being procedurally generated minecraft style, just higher fidelity. And full AI for the NPCs in the cities, cities generated on a simulation that its done, wich simulates all the history and lore of the world. Think minecraft + witcher + dwarf fortress + dungueons and dragons. Bonus points to add coop mode to get friends in your world.
Currently working on a opengl infinite terrain using voxels, wich works ATM. The AI needed would be completely insane and pretty much impossible to do at that scale, but i can have fun trying to add stuff to it.