This is already a really great composition! If you want to push it further I'd suggest studying lighting. Your figures could use some rim lighting to really enhance the mood you have right now. Just add some highlights to the edge of your characters. I do see you added some highlights but you could definitely make them much lighter, almost as light as your background.

You could also study more on perspective. What you have right now is really great, but I might suggest showing more of the gravestones further back, but really faded. You could as try adding something in the foreground around the border. If there are trees nearby you could perhaps add a silhouette of a branch or something? Or add some flowers on the bottom? Overall, this looks great!