I've noticed there is room in C.S. Lewis' Narnia tales for more stories.
Those who know the series well might think there couldn't be any more Narnia tales, since C.S. Lewis isn't around anymore and because the series as he wrote it went from the very beginning of Narnia (in The Magician's Nephew) to the very end (in The Last Battle). But there is, in fact, at least one noticeable gap in the stories, something that happened of interest that's never been written into books.
Between the time of the establishment of the first King of Narnia and the situation where the White Witch rules Narnia (that is, in between The Magician's Nephew and The Lion, the Witch, and the Wardrobe) the actions of these kings have never been documented, nor has the rise of the White Witch to power.
Since the whole series of current Narnia books is called "The Chronicles of Narnia," calling a series of books that covers the time period I'm referencing "The Kings of Ancient Narnia" would play off the contrast between the books of Kings and Chronicles in the Bible--the word "ancient" would point to the early time period of the stories. In these stories, there would be no visitors from the Earth we know, though the books could still form a children's series...focusing on Narnian children, or young talking animals even.
Or perhaps, the "Kings of Ancient Narnia" could have a more serious, adult tone. After all, the last of these books (I'm thinking there could be four or five of them) would end in a pretty dismal state--Jadis, the White Witch, on the throne of Narnia. Such a change in tone might make up for the fact the author couldn't be C.S. Lewis--you wouldn't expect a current writer to write the same way he did anyhow...
Of course, the bigger problem is that Narnia is copyrighted. You'd have to get specific permission to write books that furthered the series...which may be an unlikely occurrence. I must say though, if I were given the chance to write new Narnian stories, I think I'd thoroughly enjoy the opportunity...