Perhaps they would go on an adventure together. Chihiro might be in some sort of danger in the spirit world again, and Haku would use his powers to protect her once more. They could also meet some new and interesting spirits along the way, strengthening their bond even more.
It's possible that their meeting could lead to a deeper understanding of their connection. Haku could finally reveal more about his true self to Chihiro, and she could come to realize just how important she is to him and the spirit world. They might even find a way to bridge the gap between the human and spirit worlds more permanently.
They might have a warm embrace. Haku could show Chihiro all the new and beautiful parts of the spirit world that have emerged since she left. They could share stories of their time apart, with Haku telling Chihiro about how he has been protecting the spirit world and Chihiro sharing about her life in the human world.