Well, there are two unofficial sequels to Narnia and, Lewis himself is said to have abandoned an 8th novel.
As for Michael Ward's "Planet Narnia" theory, it makes use of both the sun and the moon. Meaning that there are other celestial bodies at work that could be used.
Do we eventually find out what happens to every character or did we have to look for the answers to some in his letters? Does he leave Reepicheep's fate ambiguous in the stories?