This is not a particularly original idea- and I'm assuming it's not already submitted , but I was thinking about the stuff that's missing, and I thought it would be cool if content could be added using a customization checklist for each world. For example...
__ Realism: ( __Stamina, __Drowning, __block gravity...) **There could be check boxes for each item. So you could check Realism and add stamina and drowning, but not block gravity,for example
__Bad weather: ( __Seasons__Rain__Snow__Freezing__Overheating...)
__Scary mobs: (Drifters break blocks, mobs that fly..., Events that affect mobs (blood moon kinds of things)
__Magic: (Spells, Potions, Items...) ** Maybe some content is all or nothing.
__Biome... & __Mountain Height... &__Ore Concentration... Or whatever you like
This way the basic game can stay pristine. And yet it can still grow and be fresh and replayable. Also, modders might be inspired to create potential content options for you as mods, if they have a chance of becoming a permanent part of the game.