The ending of Superman Returns resolves Luthor's land grab but leaves Superman's bond with Lois and Jason as the open question for any sequel.
What actually stops Luthor? Superman throws the new landmass into space, and Kitty Kowalski has already thrown away the crystals Luthor needs to keep it growing. In my opinion, that second detail matters, because it gives the villain's own crew a small hand in the rescue, so Superman is not the only reason the plan fails.
The strain nearly kills him. He falls back to Earth in a coma, recovers in hospital, and Lois kisses him. Compare that with the film's opening, where Lois has a Pulitzer for an essay arguing the world does not need a savior. The ending answers her essay without a speech: the world has just watched a man hold up a continent sized threat and pay for it.
Then comes the quiet scene. Superman visits Jason and recites the words of his father Jor-El to the boy. It could be argued that this is the film's real final beat, since the five year absence and a five year old boy invite an obvious question about who Jason is. The film does not answer it on screen, which is what I mean by an open question.
The last image is Superman circling above the Earth. Perhaps it is best read as a promise that he is staying, with the next chapter left to whoever picks it up. The series moved on with Man of Steel (2013) as the next main entry.