This title seems obvious to what will happen in the book. A battle between the school and Voldemort and his Death Eaters. I've seen this sort of thing in many fanfictions where Voldemort attacks the school. Perhaps Voldemort wants the school for himself? Perhaps he wants all the Muggle-Borns out? Whatever it is, there's going to be death...

Harry Potter and the Final Revelation

This title is a good one I think. It's doesn't give too much about the plot away (Like OotP) and gets us thinking what the final revelation could be about? A possible Harry/Voldemort link? A way to stop him. Or maybe just loose ends that Jk didn't tie up in the fifth book?

Harry Potter And The Mudblood Revolt

This is an interesting name for the book. All I can think of at the moment is that the book has something to do with Hermione or maybe something to do with other Muggle-Borns. Perhaps Voldemort decides to attack/kill the Muggleborns like in book two?

Harry Potter And The Quest of the Centaur

This title links with the Forest of Shadows title I think. I think that the Quest whatever it is has something to do with Firenze now that he has left the forest for good. Maybe Dumbledore will give Trelawney her job back so Firenze can do something more (for the Order).

Harry Potter and the Green Flame Torch

What I've heard about this is that the GFT destroys anything evil and produces good things. Voldemort would want this destroyed and Dumbledore would want this to use in the fight against Voldemort. Could it be the thing that Harry has to use to kill Voldemort? (Can't use his wand against Voldemort as they are brother wands and won't work against each other).

Harry Potter and the Forest of Shadows

Haven't heard anything other than the title which suggests the Forbidden Forest or some other forest. Things do seem to happen in forests throughout the books;

Book one - Dentention with Hagrid; see Voldemort drinking Unicorn Blood.
Book two - Follow the Spiders
Book three - Sirius hid in there and only came out for Quidditch Matches
Book four - Barty Crouch was attacked there and killed.
Book five - Harry and Hermione sent Dolores Umbridge in there and not forgetting Hagrid's brother! Also where we meet the Threstals

Other titles - Book 6 or past books?