These are some of the rumors flying around out there. These could come true, but they could also be complete lies. Who knows...

Possible Future Titles:

Harry Potter and the Green Flame Torch

Apparently there is a a torch that emits green flames that destroys anything of an evil nature and restores all good natured things. If something like this exists, Harry and/or the others would want to search for it to destory Voldemort.

Harry Potter and the Alchemist's Cell / Harry Potter and the Pyramids of Furmat / Harry Potter and the Chariots of Light

These three titles were patented by WB back in April 2000, just before the release of GoF. Either they are future book titles, are future movie titles, or were patented to throw us off the scent. I believe WB did confirm they would not be future book titles in a press conference a while back, but they might hold clues.

Harry Potter and the Forest/Fortess of Shadows

The only reference I can see here is to the Forbidden Forest, and although it holds many clues for future books, I do not think a future book will be based entirely round it. It may be that another "forest of shadows" will arise soon though. As to the fortress idea, I honestly can see no link to anything in the first five books.

If you know of any more possible titles, Send me an Owl!

Book 6 Rumors:

Professor Quirrell's brother is the new DADA teachers, and nobody like him. He supposedly wants revenge on Harry.
Peter Pettigrew is supposed to come back when Harry finds the Green Flame Torch, which kills all evil and heals all good.
Harry and Draco become friends for a while but become enemies once more after the big scary part.
Hermione might be injured in the hospital, so, once again, Harry and Ron go kick some Voldemort butt
Harry might play professional Quidditch for England.
Neville Longbottom may become a teacher.
We will find out why the Potters' are so rich.
Harry and Cho Chang will become more than friends...
Snape will temporarily become Headmaster
Lucius Malfoy will die!
Malfoy gets expelled because he did some "things" with Millicent bulstrode , and Hagrid adopts Malfoy
Harry travels to Egypt (where the Green Flame Torch is kept in a pyramid).
Bill will play a role in the plot, as he is there working for Gringotts. (JK has hinted that a Weasley will die in book 6), so it might be Bill.
Harry's eyes play an important role
Harry IS the Green Flame Torch
We find that Voldemort is truly Lily's Father (or mother for that matter)
Prof. Snape will receive the DADA teaching position, but later on lose it for an unknown reason..

Book 7 Rumors:

Snape falls in love and is also redeemed.
Lots of stuff about Lily Potter.
J. K. Rowling has already finished the last chapter, and the last word is "scar"
Hermione will be head girl, and Harry will be head boy
Harry and Draco team up to fight an "evil force"
We will find out why some people become ghosts and some do not
Peter Pettigrew returns
Harry and Hermione will have a date!
A kid asked J. K. Rowling if Snape would fall in love, and she was extremely stunned at the question and said you would find out why in book 7
Harry's father will come alive for about an hour with a potion
Harry will get to see Azkaban
We'll find out why some wizards become ghosts and some don't
Something HUGE will be revealed about Lily Potter. (The climax of the series)
Harry and Draco Malfoy will NOT team up to fight an "evil force"
Harry Potter may die and NOT become a ghost (not likely, but JK hinted towards it in an AOL chat)
Ginny Weasley Dies.
Harry will watch Dumbledore die
Harry will clean Sirius' name and go live with him.
Harry will be asked to become the Minister of Magic.
The color of Harry's eyes (Green) will play an important role
Lupin will be back again
The last Chapter may take place in Gringotts
Harry will propose to Hermione
Professor Dumbledore and Professor McGonagall used to be married and are Harry's grandparents- that's why Dumbledore favors Harry so much!
Ron or Snape will die to save Harry and Hermione
Harry's scar will disappear after defeating Voldemort. (Or the scar will disappear from being defeated by Voldemort)
"A chemistry is brewing between Ron and Hermione" -JK
We find that Voldemort wanted to kill Lily Potter because of a past relationship.
I strongly doubt this yet we may find out that Voldemort is Harry's TRUE father
Peter Pettigrew dies (In Book 3, Dumbledore told Harry that when a wizard saves another wizard, a special bond is made between them. He also told him that Someday, you may be very glad you saved Peter Pettigrew's life.")
Lily Potter and Narcissa Malfoy may be sisters. (Narcissa means "flower" and it would fit)

Know of any more rumors? Send me an Owl!