I hope that it never expires. Narnia books are clearly very popular decades after Mr. Lewis'death and I don't see why they would be less popular in the following decades. So I don't want to see poor authors trying to make a name of themselves using Narnia as a starting point.
I'm always very torn about classics becoming public domain. Sometimes new authors can be very creative and their new work doesn't depart from the spirit of the original (
Phantom, by Susan Kay, supposedly did this well), or sometimes the new story is an interesting reinterpretation of the original (
The Starcatchers Trilogy, by Barry and Pearson, gave a fun origin story for the Disney version of Peter Pan).
I'm usually less okay with new authors
continuing a story. I was not at all thrilled when Erik Van Lustbader continued Robert Ludlum's
Bourne Identity series because he retconned Webb/Bourne's backstory, killed characters that had been integral to the original plot, and otherwise ignored established canon.
However, the difference between Narnia and the Bourne books is that Robert Ludlum made a point of both telling all of Bourne/Webb's backstory and concluding the main plotline. There wasn't anything left of the story to tell, which is why Van Lustbader didn't do a very good job when he tried.
Narnia has TONS of material left to draw from. Lewis himself said that he didn't think Susan's story was finished, and there's all sorts of stories that could be told about Narnia's history. What was the reign of Frank and Helen like? How did their children found Archenland? What was so special about Queen Swanwhite and her beauty? Who was Moonwood the hare? This is, after all, why some of us write fanfiction.
I know that if there were other published Narnia books, I would read them just to see if they were any good (heck, I've tracked down four of the Choose-Your-Own-Adventure books that take place in Narnia). If the author(s) kept the Christian-based mythology rather than just making them generic fantasy, and didn't just create random children to go to Narnia without a plausible reason, I'd probably be pleased with them.
I agree with Darth Sparhawk in that I wouldn't want to see authors writing in the Narnia world just because it's already established and popular and thus makes their book easier to sell. I'd want future authors to write the stories purely for the love of the world
and to respect Lewis and his vision when they did so.