Many mods store per-world settings in the world directory. They all have a different way to do it.
I propose to add a core feature, providing a settings system that is automatically saved in a world.conf, like minetest.conf.
Mod can store everything they want in that file, with functions like minetest.world_setting_get. Or maybe overriding minetest.setting_get and similar, to get the world.conf value if defined, and the minetest.conf value if not.
And (if possible, may be a big builtin rewrite), adding the ability for mods to show a formspec just after mods loading, with the mods config, that will be stored in world.conf when fields are sent.