Many RPGs can be adjusted to fit a single player, and as a result, there are no limits to the stories that can be played alone.
The Solo Adventurer's Toolbox is a Fantasy Grounds MOD that adds in random quests, random encounters, the ability to create dungeons randomly, wilderness exploration and town settlements and NPCs. The toolkit creates random NPCs including merchants with shops, and they say infinite monster encounters.
Can you play Dungeons & Dragons by yourself? Can you play Warhammer, The Witcher and Pathfinder by yourself?