Well, that is a silly proposition to begin with, because of all the great modern fantasists, Lewis's writing is probably the least cinematic. He gives his opinions of his characters directly, he addresses the reader and he secludes most of the physical action in the background. Filming Narnia is difficult, and the folks who made the first two Narnia films did a particularly bad job of it. The Lion, The Witch and The Wardrobe's faults were mostly mistakes in emphasis and casting, Prince Caspian was both a bad film and a bad interpretation, completely ignoring the primary theme of modern man's descent into a kind of superstitious materialism. I don't plan on seeing The Voyage of the Dawn Treader, and I am very glad that Dr. Lewis doesn't have to.
But if CS Lewis had made a film, I think that it might have looked very much like Guillermo del Toro's Pan's Labyrinth. The similarities between Pan's Labyrinth and Narnia are manifold - the wartime setting, the fantastical experiences of a young girl, elders who mock the reality of these experiences, and its willingness to mix mythologies. And indeed it is the outlook on mythology that Lewis and del Toro seem to share. For the protagonist Ofelia, the world of myth is both a place of escape, and a lens that provides a stark moral view of the common world, a place where she comes to recognize the horror of her father's greed and violence.
Moreover, the fantasy world of Pan Labyrinth is not merely story, it is, from my view, a coexistent reality (I have heard arguments that the fantasy adventures are merely Ofelia's imaginings). Perhaps the most original of Lewis's ideas was his claim that Christianity was "myth become fact," the historical incarnation of ageless redemption stories. This idea is certainly present in Pan's Labyrinth. The film's mythology is not explicitly Christian, but it is highly suggestive of the Christian narrative. Without giving away plot details, it includes a spiritual being taking on human flesh, sacrificial blood as a source of redemption, and restoration to right standing before one's father and king. Interesting, del Toro claims that his film was intended to subvert Catholic dogma; a Catholic friend of his called it a "deeply Catholic film". It surely offers much better spiritual food than the newest Narnia blockbuster.