Very cool topic! I read through most of the other replies, and I think you all have amazing ideas :)
I especially love that idea, ReisenFanatic :D
Feels like so many games these days are about violence so that sounds cool.
Me personally, I always wanted to make a big open world game (map is like 2 km wide) in which there is no real objective except survival. You get to start at a human village, and try to make friends or enemies or whatever.
I am thinking about a HUGE emphasis on npcs that adapt to the environment. For example if you help out certain people, their enemies start to hate you. Or if crop yields are getting low, villagers start blaming each other. You can even spread rumors that people may or may not believe! It's probably nearly impossible to make such a complicated system, but I like to imagine :)
Anyways, the character and settings.
Since I only play touhou 4, the main character is mountain youkai Orange. She might pass as a human to most, but needs to be a bit careful near people like Reimu.
Overall nobody is really hostile at the start, but environmental factors (famines, rain, earthquakes), player interactions, and npc actions slowly shape relationships.
It all depends how you react. You could be a helpful person that is friends with everybody. You could be a secret murderer that the police keep struggling to catch.
It's your game, your world, your life!
Writing this during class so I might be making no sense right now hehe. Thanks again Yumetou for the interesting question :P