Even though there isn't an actual movie sequel, how would Haku stay true to his promise and meet Chihiro again? His river is mainly filled up, with apartments on top of it, and that is his connection to the human world. Sure, he stopped being Yubaba's apprentice after Chihiro left and got his full name back, but how would he go see her again? Is he allowed anywhere in the human world besides his (former) river? And if they somehow met up with each other, would Chihiro's memories of everything come back (eventually)?