The Chronicles of Narnia series which was comprised of The Lion, the Witch and the Wardrobe (2005) Prince Caspian (2008) , and the Voyage of the Dawn Treader (2010) was a very successful series, grossing over $1.5 billion worldwide. 
Fans of the books, however, know that there are more than 3 books in the series, and I am sure an adaption of The Horse and His Boy would be as successful as the others. 
Since all but one of those who played the main Pevensie characters have continued to act, it would probably be ideal to have them reprise their roles, especially as they would now be at the right ages. 
There are many, many benefits to making a film like this.

For a start, the storyline is all set out, and it’s a good plot, too- there aren’t too many descriptions and has enough action to make an exciting and interesting film for children and adults alike. The characters are well developed and described as well, each of the main ones (Shasta/ Cor and Aravis) are given backstories and descriptions of their personality traits. 
And the scenery would be good too. There are deserts, and fields, and mountains and rivers. A whole range of diversity, which, with the right cinematography, would be stunning.

It would hopefully bring in some attention to the lesser-known books, which have amazing plots and, (as far as I’m aware) there aren’t any film adaptations of them. This would bring in a wide range of audiences- from those who already know the books, to those who have seen the films and wish to delve deeper into the beautiful and complex world of Narnia.

I myself am a huge fan of Narnia. Despite what some may think, the fandom is actually bigger than one might expect. I believe that the lesser known stories deserve more fame, and that their stories are just as (or more) interesting and dramatic as the ones that have already been adapted. 

The Horse and His Boy follows the story of a young boy, names by his master as Shasta, who steals a war horse (names Bree) and runs off to Narnia to avoid slavery. Along the way, he meets a girl names Aravis, who is also fleeing to Narnia to get out of an enforced marriage. However, along the way, they are separated and Shasta is mistaken for the Prince of Archenland- Corin.

Meanwhile, Aravis learns that the Prince of Carlomen is planning an attack on Archenland and Narnia. When they meet up again, they have to race across a desert to reach Archenland in time to warn them. However, they are chased by a lion and Aravis is wounded. Shasta has to run the rest of the way to King Lune of Archenland, but gets lost along the way, receiving guidance from Aslan. 
He finally reaches King Lune of Archenland, and tells him of the attack. 
After the battle, which was won by the Narnians and Archenlanders, Shasta learns his real name is Cor- he is the long lost twin of Prince Corin, and that he will someday rule Archenland.
In the end, he marries Aravis and together they rule Archenland. 

This is only a brief synopsis of the story- which I am sure would grasp the interest of many fans, as well as attracting new fans of the already popular film trilogy.