The phoenix in this title is very likely a reference to Fawkes, Dumbledore's pet phoenix, whose tail feathers are imbedded in the wands of both Harry and Voldemort. What's going to happen in book 5? Here's a nice list of quotes by JKR herself which give you some idea. The list has been circulating in various fan groups online.
What will happen in Harry Potter and the Order of the Phoenix?
All original JKR quotes, with sources at the end:
� "In book 5, we go into a whole new area, physically, an area you've never seen before, a magical world. What's very important for me is when Dumbledore says that you have to choose between what is right and what is easy. This is the setup for the next three books. All of them are going to have to choose, because what is easy is often not right."
� "You'll be seeing Mrs. (Arabella) Figg in book five, and you'll find out all about her. Harry will see the real 'Mad-Eye' Moody in book five."
� "Why do some wizards or witches become ghosts and others don't? You will find out much more about that in book five. There will be a female Defense Against The Dark Arts teacher."
� "You see Lupin in Book Five. Yes you see a lot of old characters in Book Five. Book five will be scary. Harry finds out a lot of things he hasn't stumbled across so far."
� "The Dursleys are in the next book, and there's stuff coming with them that people might not expect. Ginny Weasley (little sister to Harry's best friend, Ron) will play more of a role in Book Five.
� "In book five, Harry has to examine exactly what death means, in even closer ways."
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