Well, lets see if maybe we can figure the best guess by going over what we know:
- Each book title is an object/person/people which is the core of the plot but not always the main topic.
- With the exception of Chamber of Secrets, no book has been a location.
- Four of the books have included the word "of" in the title
- No titles have included transportation
-The Chamber of Secrets, Azkaban, Phoenixes, the Goblet of Fire, and the Sorcerer's Stone are all very old.
I think it'd be safe to narrow out Furmat.
Sooo, my best guess would be Harry Potter and the Alchemist's Cell

I just think it flows best.
But I'm not putting Chariots of light out of the realm of possibility either.