The Deserts of Isis

You probably didn't need me to figure this out, but Isis is the stand-in for ancient Egypt in Dragon Quest III's overworld. It's located in the largest stretch of desert in the game, corresponding to the real-world Sahara, around an isolated oasis. Actually getting into Isis is somewhat of a minor puzzle - you have to enter the oasis from a specific direction in order to have access to the city. This, to me, suggests that the capital city is difficult to reach, with a natural moat creating a strategic checkpoint around the entrance. 

One could, if they wanted to, play Isis as a lost civilization, isolated from the rest of the world by inhospitable desert. The place definitely has a more ancient vibe than anywhere else in the game world - however, as we've established, the world of DQ3 is very much History's Greatest Hits anyway, so it doesn't really seem that out of place when ancient Rome and feudal Japan exist in the same world. Furthermore, Isis seems like a fairly powerful kingdom. It has its own queen and an extensive palace, and an arena similar to the one in Romaly. There are also NPCs there who reference having come through the desert, suggesting that it does get foreign traffic. With that in mind, I'd be inclined to theorize that only the capital is difficult to reach, and that Isis actually controls a number of cities and settlements throughout the desert, though the oasis it sits in is likely an uncharacteristically fertile spot that allows for enough crops to be grown to sustain the population (fittingly, you could compare this to the Nile Valley). It's also located fairly close to Portoga, which could imply the existence of important trade routes. All of this could explain why such a prosperous kingdom is located in the middle of the desert.

Isis is clearly a land with a rich history. There's a pyramid located to the north, which forms one of the largest dungeons in the game. Even by the time the game takes place, the pyramid is clearly an ancient one; the NPCs regard it as an abandoned ruin rich with treasure and traps alike, where the bones of bygone kings are buried. Clearly it's the remnant of an older civilization - paralleling how Cleopatra was closer to us than to the pyramids - and there could very well be all manner of ancient ruins from the same era scattered throughout the desert as well. The pyramid is the location of the Golden Claw, one of my favorite items in the game. It's one of the best weapons available, but once you pick it up you'll be constantly attacked by the undead until you either leave the pyramid or drop it. A powerful magic item that attracts enemies would definitely be a fun trap to include in a dungeon, forcing the PCs to evaluate the risks and benefits of keeping it.

Art by Nassr

It seems that Isisian funerary traditions continue in some form to the present day as well. There is a system of tombs under the palace where treasure is buried - one such tomb is haunted by a ghost who appears if the treasure is taken, but is willing to let the party go with the treasure if they answer honestly when asked if they took it. I feel like Isisian tombs, both ancient and modern, could offer a lot of potential for dungeons, with all the traps, treasure, and haunts they entail.

Considering that Isis exists alongside ruins of a more ancient civilization, and it's ruled by a queen renowned for her beauty, the closest real-world era would be Ptolemaic Egypt. A DM could lean into this by incorporating ancient Greek influences into the kingdom alongside the Egyptian ones. It's worth noting that one of the townsfolk is a man named Sokras who spouts philosophical musings that others find eccentric, like "I'm waiting for nightfall. But when the night comes, I long for the morning." I didn't make this connection myself, but apparently this is supposed to be a reference to Socrates. There's just as much precedent, if not more, for putting hoplites and agorae in Isis as in Romaly. If you're concerned about Isis feeling too much like a copy-and-paste version of Egypt, incorporating more Greek elements might help set it apart.