If I remembered correctly, I read somewhere in Michael Ward’s Planet Narnia that originally C. S. Lewis did not plan (or had not planned) to write seven volumes of Narnia. His original goal is, to put it in Tom Wright’s words, to rediscover Jupiter in our Saturn-dominated late modern age, where Saturn connotes gloom and death and Jupiter embodied joviality and royal joy. And that’s why he wrote The Lion, the Witch, and the Wardrobe (LWW). It tells a story of the Pevensie siblings who were evacuated from London due to the ongoing World War II — a dark and gloomy setting. Then they discovered another world in a wardrobe, which, unfortunately, was held captive by Saturn as well as it was “always winter but never Christmas.” That is, before Aslan came, redeem, and revitalize Narnia, and thus indicated the Saturn-to-Jupiter turn in this volume (it explains the mumbo-jumbo of mythologies in Narnia, e.g., Santa in LWW, as what’s more important for Lewis is the spirit embodied by these characters). Thus LWW will suffice on its own if Lewis simply wants to achieve his aim to rejuvenate his saturnine world.
But then he wrote another volume, and so on until he completed the whole series. Nevertheless, one could argue as well that the first three volumes of Narnia could form another independent series on their own: LWW, Prince Caspian (PC), and The Voyage of the Dawn Treader (VDT). Indeed, they have been called the Pevensie trilogy, as the main protagonists in these volumes are the Pevensie siblings (although only Edmund and Lucy featured in VDT). They also ended nicely in VDT, as Aslan told Edmund and Lucy that it will be (perhaps) the last time for them to enter Narnia.
“Please Aslan, before we go, will you tell us when we can come back to Narnia again? Please. And oh, do, do, do, make it soon.”
“Dearest,” said Aslan very gently, “you and your brother will never come back to Narnia.”
“Oh, Aslan!!” said Edmund and Lucy both together in despairing voices.
“You are too old, children,” said Aslan, “and you must begin to come close to your own world now.”
Edmund and Lucy has grown since their adventure in the LWW, and they might be too old now for Narnia (in VDT, Edmund is 12 while Lucy is 10 although they are a bit older in their cinematic incarnation).
The ending in VDT is also appropriate for a grand ending. Reepicheep sailed into Aslan’s Country. Regardless of whether we agreed with Lewis that the goal of creation is to reach Aslan’s Country (and that perhaps it has popularized the belief among Evangelicals that heaven is our goal of existence), it fits with the Narnian worldview. They have reached the very end of Narnia, the border of Narnia and Aslan’s Country.
Coincidentally, the movie franchise of The Chronicles of Narnia might end here as well. The harsh world of commercialism might not allow completion of the series, as the generated income from the movies kept decreasing from LWW to VDT (LWW: US$750mil, PC: US$420mil, VDT: US$330mil) and thus might not warrant production for further volumes to be filmed. I will be very happy to see myself wrong, though, but it seems to me that the Pevensie trilogy might be enough for this franchise. Indeed, Aslan himself told us in the end of VDT that, although we, like Edmund and Lucy, can’t see him again in Narnia, we shall be meeting him somewhere else. Here, indeed, although here he is called by other name.
“It isn’t Narnia, you know,” sobbed Lucy. “It’s you. We shan’t meet you there. And how can we live, never meeting you?”
“But you shall meet me, dear one,” said Aslan.
“Are — are you there too, Sir?” said Edmund.
“I am,” said Aslan. “But there I have another name. You must learn to know me by that name. This was the very reason why you were brought to Narnia, that by knowing me here for a little, you may know me better there.”
This was the reason why we were brought to Narnia, not so that we end up contemplating Aslan forever in Narnia, but that by knowing Aslan there for a little, we may know him better here. Thus The Chronicles of Narnia function as a pointer and a witness, like the pointed finger of John the Baptist in the Grünewald Isenheim Altarpiece.