After Haku remembers he is the Kohaku River spirit and Chihiro wins Yubaba's final test, he tells Chihiro that he will stop being the witch's apprentice. However, since his river has been filled up to put apartments there, what will he do now? Even though there isn't an actual sequel, what would he do? His real home has been filled up, so unless he can find some way to get under the apartments and find water there, would he just stay at the bathhouse? He wouldn't exactly be "in danger" from Yubaba there, since he has his real name back and the black slug she used to control him is gone, but would he even want to be there? Where would he go? And would he actually become a customer every now and then at the bathhouse, like other spirits?
Maybe, just maybe he went and search to look for another river? Who knows. But he did said he's staying at the inn after sending Chihiro off, probably still works at the bathhouse, banishing or co-work with Yubaba?