There are now three of those films. However, the third one just isn't really based on anything Agatha Christie actually wrote. They say that a haunting in Venice is loosely based on Halloween party by Agatha Christie, but I've read that book and there are no ghosts and it does not take place in Italy. It's literally just a Halloween party in England where some kid happens to get offed in the barrel where they were ducking for apples

When I was actually watching the movie of course I wanted more. I looking at the progress bar at the bottom of the screen realizing that the movie was almost over, and honestly not being sure if I would get any satisfaction. I was ready to be extremely disappointed and sad if they had never met again. However, despite the fact that I definitely would love to see more of their story, I just don't think a sequel would make sense. They've had their adventure, just let them live now. Also, while I don't think shinkai would ruin anything, sequels have a nasty habit of just mes a story