I have just finished rereading Harry Potter and the Goblet of Fire, and have the following thoughts heading into Order of the Phoenix:

Almost lost in the fervor over the return of Voldemort was the potential for conflict with the wizarding government as represented by the Ministry of Magic, which in Goblet of Fire was revealed to have the same general ineffectiveness, corruption, scheming ambitious bureaucrats, and infiltration by the enemy found in places like the Galactic Republic from the Star Wars prequel trilogy or Earth-Gov from Babylon 5. It seems likely that conflict will escalate between the Ministry and Dumbledore, especially now that some of its trusted members will be working for a returned Voldemort. The fact that investigative reporter Rita Skeeter is out of commission for the next book also creates room for real scandal to develop inside the ministry. This could go pretty far...especially if they return to the police-state crisis mode hinted at in the discussions of Crouch's policies during Voldemort's first rise, though Fudge's refusal to admit Voldemort's return makes that uncertain (or him potentially expendable?). Still, anyone up for an HP version of the Rush Act? I'm not sure what effect this has on my speculation about who will die. The threat to Arthur Weasley is elevated, and technically Percy, whom I believe will have an important role, though I suspect he'll be more involved in making key character choices of some kind. (Admittedly the death thing could be just someone who happens to be randomly in the wrong place at the wrong time, like Jadzia Dax from Star Trek: Deep Space Nine.)

Several times in Goblet of Fire, JKR led us to wonder why Dumbledore trusted Snape. That will probably be answered in Order of the Phoenix. My hunch is that Dumbledore's trust is well-placed: Snape is certainly ambitious and spiteful - he's similar to Raistlin Majere from the Dragonlance Chronicles in that respect - but I somehow will be disappointed if he returns to the Death Eaters. Voldemort also said he suspected one of his followers was lost for good - assuming Karkaroff was the one who fled, Snape is the one they're going to kill. If he's planning to feign a return to Voldemort to spy on them, that might complicate matters. And if Voldemort does go after Dumbledore, one way to do it would be the Dr. Yueh route from Dune.

I'm not sure were the Durmstrang thread is going. We have an entire school of people who know the Dark Arts, but we don't know enough about the wizarding world to know exactly what that means. It might become a secret stronghold of Voldemort, or it might become potential recruiting for good guys who know the Dark Arts, or it might just fall by the wayside. Viktor Krum at least seems generally on the good side, which raises interesting possibilities.

Fleur Delacour indicated she would seek a job at Hogwarts. Which? If Hagrid is off looking for giants or if the Ministry has forced him out, she could teach Care of Magical Creatures. There's also an opening for Defense Against the Dark Arts, unless Moody comes back. Will JKR pick up on the thread where she was eyeing Bill? Jordan told me JKR has indicated a key to the series is a pairing no one has picked up on yet. Fleur is an extremely powerful witch, and hence could be part of such a pairing, though I figure it has to involve one of the Big Three and I can't see them suddenly taking up with a school employee, unless Dumbledore is killed and replaced as headmaster by Stephen Harper =)

Anyway, once I start, I'm going to be reading in slow appreciative mode, so no spoilers please. I should be done by this weekend, and promise that no plot details will be posted to this blog.
UPDATE: Jonathan Edelstein now has a post called "Harry Potter and the War on Terror" which I fled due to the spoiler warning, but the subject line to which suggests I might be onto something in the above =) Of course, now I know not to read his site until I finish the book, just in case my eyes wander somewhere. I already ran into a spoiler last week from CalPundit, but that just told me that the students continue to find history boring.