Legendaries could have subworlds or some implementation of them + a cutscene or cool loading screen to add to the feel of fighting...yknow, a legendary
this has been considered for some specific legendaries like giratina. your idea is a nice extension but depending on the implementation it might be too bloaty. the alternative way is making custom structures and slapping those into the main world but terraria worlds are so tiny that it'd be kinda awkward. we need to talk more abt these ideas later so we can achieve a good impl