i already think adding star coins is a handful but should i also make switch palaces like in mario world? if so please tell me how it could go wrong so i prevent tiring replayability!
up to you. do you want to? go ahead. if not, feel free not to.
not much can really go wrong if you do decide to have them. see various smw romhacks and how they use them: secret exit, making platforming easier, sometimes making platforming harder (prominently)... you can do what you want