The Vampire Diaries/WMG

Elena is Katherine.
Elena looks exactly like an immortal vampire from the 1860s, and survived a car crash that killed both her parents. The simplest explanation is that she actually is the vampire, and "Elena" is just her cover identity. As for the sun, maybe she just has an earring that protects her.

Lexi is an Original.
Because:
 * A) She is so popular that the writers will want to bring her back, and flashbacks alone aren't enough for that.
 * B) It would be awesome.

The First Woman is Klaus’s mother and the First Doppelganger is his sister.
Klaus’s mother was a witch who became a vampire by drinking baby’s blood. She was a lamia, a vampire who could have babies and she turned Klaus’s father into a vampire as well. Her daughter was a doppelganger because she was a girl. Klaus’s sister had a baby with a human and that diluted the lamia blood, so the next doppelganger turned up some generations after, increasing in generational span until Katherine, and then Elena.

The "ghosts" are in fact the First Evil.
They share the exact same attributes as the First - incorporeal, can only appear as dead people. In 3x06 Vicki attempted to become corporeal, the First's exact plan in Buffy. Vicki's plan seemed oddly OOC, far more on the evil side of the spectrum that alive!Vicki was. Explanation: it's not Vicki, it's the First Evil playing on Matt's insecurities.

Tyler or Caroline will die in the Season 3 finale
Now that they have set up a very good reason why one or both of them would betray the others and save Klaus one of them has to go. Either Caroline simply because Klaus get bored of her saying now triggering Tyler to want his vengence no matter the cost or Tyler because Klaus doesn't like competition. Either way unless some third option presents itself one of those two isn't long for this world.
 * Partially Jossed.

In order to defeat Klaus...
Elena and co. will revive all of the Original family. Elena will obviously try to rescue Stefan, and will realize that she doesn't have the firepower to even hope to touch Klaus. So she will track down Elijah (probably using Bonnie's tracking spell - remember Bonnie needed Elena's brother's blood to find her, and now they have Klaus' (Elijah's brother) blood). Elijah is buried along with the rest of his family, so when he wakes up he will then revive all his brothers and sisters. Cue 6-on-1 Orginal-Vampire sneak attack.
 * Damon is the one who ends up undaggering Elijah who then undaggers his siblings and cue the beat-down

Matt is doomed.
The show doesn't hesitate to kill of supporting cast members and likes a body count for finales. Matt thinks that Caroline killed his sister, which makes him rife for being used by any number of villians, and he doesn't serve much of a role on the show other than being Caroline's love interest. He's got Alas Poor Scrappy written all over him.

Alternately, Bonnie is doomed.
The show has gone two seasons and failed to do anything with Bonnie. Matt could at least fill a Xander-esque role as the only real Normal on the show and now that, he will probably get some more interesting storylines. While Bonnie on the other hand has even less of a character than Matt, and it seems like she will be sidelined in her own romance storyline what with the return of. Plus, her new powers have made her Too Useful To Use, as she is basically a walking Deus Ex Machina now and so the show runs the risk of relying on her too much to easily solve plots. Alas Poor Scrappy is more or less the only option left for her. Either way, the show is bound to kill off another main character sooner or later, and Matt and Bonnie seem to be the most likely candidates.
 * Not to mention that Black characters seem to die a lot in the show

Elena will end up with an Third Option Love Interest.
Just to make the shippers squirm.

Caroline will develop feelings for Klaus.
It's as simple as that. When Tyler is revealed as still being alive he'll either come up with an explanation. Either the hybrids simply survived because they weren't vampires, and thus weren't subject to the same rules for some arbitrary reason or he'll reveal that Klaus somehow survived. From there it just writes itself until the truth one way or another comes out.

Katherine will be the one to help Elena transition
Stefan is still not so good around human blood, and Damon makes a poor mentor. The originals will probably be leaving town, and Caroline is dealing with her own problems. Katherine will swoop in and help her transition into a vampire -- not without an ulterior motive of course. She'll earn points with Stefan, and it never hurts to have another Vampire that looks exactly like you when you have as many enemies as Katherine has.

Klaus is The Great Gatsby
Think about it - he was alive in the 20s, is fabulously wealthy but with a Dark and Troubled Past. He didn't actually die at the end of the novel, he just faked his own death because his father was onto him. And Klaus is certainly into making big romantic gestures and wearing smart suits. That's why he's so attracted to Caroline - she reminds him of Daisy, but is also caring and strong (unlike Daisy).