The book ends with the murder mystery neatly solved and the children looking forward to going back to Egypt once they've spent Christmas with their families. But in the last paragraphs, April poses a question:
They lay there, staring into the future gloomily for a while, their chins on their hands. Then April turned towards Melanie, slowly and thoughtfully. "Melanie," she said, "what do you know about Gypsies?" (23.56-57)
This question sets things up perfectly for a sequel (which exists!). Because the kids have pretty much exhausted all things Egyptian, it makes sense that they would pick a new interest for their next game. And it might as well be gypsies.