Hmm...
Possibly. I mean, up until VDT, all we knew was there was the extension of the Lone Islands, Calormen, and Archenland.
Maybe it's like in VDT. Possibly. We'll never know... C.S. Lewis left it to us to think up.

We know there are also Galma and Terebinthia. Telmar is also a country but I'm not sure if it was populated during LWW. There might very well be something south of Calormen. The kings and princes could be part of smaller countries, maybe islands north or south of Narnia. Of course Lewis could have just written because it sounded good and then forgotten about it. In my mind I’ve picture them coming mainly from the general direction of East (i.e. over the water.) There could be countries farther west as well. I guess we can imagine it as big or as small as we want.

I never used to imagine there being other countries, in fact the first time I read the books and I came to HHB, I was all confused about these other places lol. But to answer the question, yeah I think so, it would make sense.

Well, go each direction and see what you find. C.S. Lewis covered all the way East in Dawn Treader so we know what there is that way. The village Shasta lives in is south of Tashbaan. But Bree's master lived even farther south. And Rabadash says that Narnia is even smaller than the Tisroc's smallest province. So even if there aren't any other countries farther south, there's a whole lot more of Calormen.
Go North and there's Ettinsmoor and Harfang. The White Witch came from the north. And so did the Lady of the Green Kirtle. There could be a settlement past Harfang.
Go West and you wind up in the Wild Woods of the West, beyond that are the ice-capped mountains and green valleys that Digory, Polly, and Fledge went through on their way to the Garden. Telmar is to the west. I'm not sure how far though. Oh! There are people to the west! In the first chapter of the Last Battle, the lion's skin that Shift finds floats down the waterfall into Caldron Pool and the narrator says that it wasn't a talking lion, and that a hunter had shot and skinned it.

Yes their probably is.Their should be anyway. Like all the island they visit in VDT.

There must be. Like everyone else has pointed out, there are a couple of references to other places.

Hmm.. interesting thought. I can see myself writing stories set in other places around Narnia..

There's the Wild Lands of the North, Ettinsmoor, and Harfang. There's also Aslan's Country and all those islands that they go to in VDT. And there's the Underworld which was a huge land and there's Bism.

Pattertwig's Pal: Telmar was, in fact populated during the Golden Age.
And the end of LWW mentions 'kings beyond the sea', I think. So possibly there is another continent somewhere. I think, though, we've got all the countries on the Narnian continent, as well as the islands.