is this really the end?

Yamada's mother came to Ueda at the end and told him that Yamada would contact him, even if she was dead. "Even if a person dies once or twice, they will always remain the same" is said afterwards. In the end scene, Yamada appears - very much like Sadako from "Ringu" - with lowered head and hair over her face. That is a strong indicator that she is a spirit. Then she performs a trick, because spirits keep on doing what the living person did before. That's why Ueda is moved to tears.

In summing up, there was the possibility of a sequel, but as a living/ghost duo, IMHO.