Some of this idea can be done with a custom map. the mobs, dimensions, anything else "new" can be done with a mod/mods. It could work like you play the world normal at first... then you get teleported to a new dimension after you beat the enderdragon and wither that would look just like your old one. but... everythings changed! your house is still there, but new mobs have appeared, teleports to other dimensions have appeared, etc!