|music||:||Mozart - Symphony No. 35 in D Major|
There are tons of theories out there, but these are mine. Others might have independently come up with the same thoughts. The worlds/realms that fiction creates are fairly constrained. I wouldn't be at all surprised if others have thought of the same things as I have.
This post is a living document. I add to this when I think of it instead of posting a new entry for each one. Sometimes, there may be things that don't flow because I've added to it over time.
For reference, I've read all four books and follow The Rumour Mill® very closely.
Voldemort's Maternal Family
I somehow get the feeling that the Evanses (Lily Potter's maternal family) might be related to the Riddles ... somehow. The Evanses and the Riddles may be blood relations or just family friends or there may be some kind of drama between the two.
We haven't found out yet what Voldemort's mother's surname is. We know that his middle name, "Marvolo", is in honour of his maternal grandfather, but I had the impression that "Marvolo" is his grandfather's first name - it just sounds more like a first name in the Harry Potter world. Maybe because "Marvolo" doesn't sound native to the UK...
If Tom is the Heir of Slytherin then it might make sense that his maternal family side surname is "Slytherin". But in British culture, an heir can be the son or daughter of a son or daughter. It's not like Chinese culture where an heir can only be the son of a son (if there are no sons, there is no heir). If it were only sons, then you'd always have the family name as a confirmation of lineage. But as soon as you put a daughter in there, you don't have that obvious association.
So, there's an off-chance that Voldemort's maternal family could be the Evanses(!!!) Perhaps they decided to turn their back on the Wizarding world after how Voldemort's mother was treated by the Riddles (or it could have been for a multitude of reasons and this could have been just one of them).
It may be a huge coinky-dink, but huge coinky-dinks have been happening all over the place in the series.
Harry is a Gryffindor-Slytherin Mix
Supposing Voldemort's maternal family is the Evanses. There have been theories that theories that Harry is a descendant of Godric Gryffindor - "only a true Gryffindor can summon Godric's Sword." Supposing that is true as well.
Then Harry is a Gryffindor from his father's ancestry and a Slytherin from his mother's side. This might be why he has qualities of both, which is why the Sorting Hat had difficulty sorting him and even stands by its initial statement that he would have done well in Slytherin House.
The Ties That Bind
It seems like, in the Harry Potter Universe, malevolent Magic and Spells don't work the same way when cast on family/blood relations.
Harry's and Voldemort's wands were Brother Wands. They were made out of two feathers from the same pheonix. They won't do what you expect when they go head to head in duel.
As an extension to that reasoning, then, if Voldemort is a blood relation to Harry, it would make sense that Voldemort's "Avada Kedavra" on Harry backfires and actually works to bind their lives and destinies even tighter than before.
But, why he's successful in killing Lily, then, I don't know. Of course, others have speculated that Lily is not actually DEAD dead. JKR says we will never see a live Lily or James Potter, but that doesn't rule out that she can be an Animagi stuck in animal form or a ghost or something.
Snape & Lily
Another theory I have is that Snape was in love with Lily Evans when they were still students at Hogwarts, but Lily fell in love with James. That's why Snapes hated James Potter, Remus Lupin, Sirius Black and Peter Pettigrew. Or that's why Snapes hated James Potter even more than he originally did. And now, he has to be reminded of that every time he sees "a little James" running around.
Who will die in Book 5?
JKR announced that there would be a major death in Book 5 - a "fan" of Harry's will die.
JKR wants to make an impact. Whether it's to solidify her celebrity status after the series ends or the more philanthropic purpose of exposing youngsters to death and learning to deal with it, it needs to make an impact on the audience.
It needs to be someone prominent and not someone that the audience might have indifferent, mixed or negative feelings toward. That just wouldn't work right.
That rules out Dobby (this manipulative, meddling, passive aggressive little rat needs to get turfed), Ginny Weasley (she really hasn't done much to make us really feel either way about her), Bill, Charlie & Percy Weasley, the Creevy brothers, Severus Snape, Sybil Trelawney, all three of the Dursleys, any of the Malfoys.
JKR also said that writing this death was "difficult for her". I think this rules out Dumbledore. I don't think killing off Dumbledore would be particularly difficult. There's a certain peace that comes with the death of an elderly person, that counters the sadness and sense of loss. They've had a great life and touched many people along the way, everyone loved them, their great achievements will always be remembered. Everyone thinks, "well, maybe it was his/her time to go".
It would be a much more heart-wrenching for someone young or in their prime and well loved by all, to die. It's seen as "needless", "unfair" and just plain sad that fate/destiny would take someone that held so much promise. I was in tears at Cedric Diggory's death at the end of Goblet of Fire. Not so much at the time that he was killed, but when Dumbledore said at the year end feast, "Remember Cedric... Remember Cedric Diggory".
This leaves (in order of impact): Hermione Granger, Rubeus Hagrid*, Ron Weasley, Arthur & Molly Weasley, Fred & George Weasley, Sirius Black.
* - I have mixed feelings about Hagrid. I know he's well-loved and was a great help to Harry just before his first year, but why the hell did he leave Harry at King's Cross Station without explaining how to get to Platform 9¾?! If the Weasley's didn't come along, Harry would never have figured it out.
Hagrid just strikes me as very irresponsible and unreliable. He is irresponsible with creatures - buying baby Norbert, spilling beans to the Quirrel in disguise who sold the Norbert egg to him and leaving the children to clean up after him. He's has a serious discretion problem. And at times, I just think he's plain dangerous.
I don't know for sure, but I think Mandrakes are based on Ginseng in real life. Ginseng roots have always looked like little people to me. =) poufywerewolf pointed out to me that the Mandrake plant (Mandragora officinarum) was the basis for the Mandrake plant in Harry Potter. But when I Googled for images of it, the Ginseng plant looked more like little people to me.
This has absolutely no bearing on anything, just something that occurred to me.
Massive book and movie spoilers as well as future storyline speculation in the TALKREAD...