I'm not sure if this has been done or proposed before, but here is an idea for a challenge:
You are part of the indigenous people of the land. You live in harmony with nature. Your goal is to beat the game but there are certain limitations, because you can only use what nature provides! So…
– no cutting down trees! (picking branches and cutting dead trees/stumps is fine)
– no killing of animals! except they attack you first or you need the food for a boss fight! (you have to respect them and not agro them on purpose)
– you HAVE to kill every evil spirit you see to put them to rest! (bosses, skeletons, ghosts, firey boys)
– you are not allowed to enter or disturb any graves!
– (optional) sit and speak a prayer if you have to kill animals
– (optional) only build tent-like structures
That's all I've come up with so far. Any additions are welcome!
Edit: some worlds might not work if there are absolutely no fire demons because you wouldn't be able to get cores for furnaces!