Pandora Hearts/WMG

When Vincent encouraged Gilbert to make a contract with the Raven, it was part of a Plan designed to prevent
Specifically, if Gilbert has a contract with the Raven, it means that it won't be possible for

Elliot is going to be the one who is possessed
First off, he "composed" Lacie, second, he's a Nightray, who have an ambiguous background as far as black and white good verses evil and third, those nightmares? Yeah. Just flashes from the past.
 * And Jossed.
 * And Jossed.

Oz is The Wizard of Oz.
Because y'all are taking this way too seriously. (Also, Oscar is Hohenheim.)
 * Also, similar to Hohenheim, Oscar had an ailing wife.

Leo is a woman
You can't tell me that you haven't noticed his womanly curves.
 * FINALLY, someone agrees with me!!
 * You know what happens when the glasses come off, the hair is brushed, and the clothes are tighter...
 * His eyes really are pretty after all.

Gilbert is/was/will be the Big Bad.
Yeah, I know, it's PoisonOak, but it's just an idea that I got while rereading some of the old chapters. Vincent is constantly talking about how he does everything for Gil, especially in the flashbacks to Sabrie. Gilbert has no idea what he's talking about... but Gil has also lost his memory. What if Vincent really was/is doing everything for Glibert, in the sense that Gilbert actually told him to do all this before whatever caused the memory loss? If this is true, he will at some point regain his memory and resume his place as the Big Bad, Light style. I'm admittedly not caught up with the current story line, so if there is anything that has happened in the later (or earlier) chapters that debunks this theory, please by all means send it the grave.
 * This other troper also strongly believes he, not Vincent, was the one who killed "Alice" in the past - but that part is still very fuzzy.
 * Jossed:

Leo is the Head Hunter/
He's the oldest child we've seen from the orphanage, he has more access then most people to the Nightray household, and there's certain lapses of memory that Elliot can't account for. The reason Elliot isn't dead yet is because Leo is attached to him, and is disregarding whatever reason he has to kill everyone else. There's also the fact that Elliot has commented on his eye color, something that we have never seen. Two other character with "unusual" eye color are Barma and Isla, who are both from another country. We know that Isla has connections to both Leo and the Headhunter; could it be that Elliot has been a plant the entire time?

Vanessa is Celty Strulson.
So Vanessa. So I'm thinking she and instead moves to Ireland where she spends some time as a Dullahan. And changes her name to Celty Strulson so that no one who knew her as Vanessa Nightray would know.

The power of
As of now, we haven't seen the power of before. And even Break have specifically said that that Chain is not specialized for battle. Then what it's specialized in? Healing! We all know that in the original story,  is  to Mad Hatter. And to elaborate;  power can only be triggered if there is Mad Hatter nearby. So in way or the other, with the help of Break,  again And The Fangirls Will Rejoice.
 * I am so waiting for this to happen. Bonus points if it happens in the middle of the fiery hell that is
 * Aaaand as it turns out.

Alice
Just bear with me guys, this theory is as random as they come: We know that Lacie long enough before for  not to know who she was, before. It can be said that the Will of the Abyss more or less makes the Abyss "stable"( or as stable as it can be. I got the impression that before the Tragedy, there was nobody to actually control the Abyss, so it was actually wilder than what it is now, it's time-bending, soul-twisting abilities included. I also remember that it was said once in the manga that . You may see where I'm going with this.

Somebody (probably Glen) tried to bring her back, but by that time, the Abyss had already gotten to her, deforming both her appearance and mind, although not completely. And what we have when the "Chainification" is left halfway done? BAM! That would explain why Alice didn't change all that much physically (giant rabbit form notwithstanding) from human to chain, her body was already previously modified. And thanks to the Abyss not having someone in charge, it went bananas with her mind, wiping her mind clean of her previous memories, leaving a teenager body with the mentality of a little child. Somehow, she and her "chain-half" became deeply bound to the Abyss, which turned her into a valuable asset to have. But her resemblance to Lacie was so disturbing to the Baskervilles that they put her into that tower, to avoid having any sort of contact with "such a mockery of that person".
 * If that's so, then it's also possible that B-Rabbit!Alice is the Lacie Glen wants back. After all, the present Alice is rather different from both black and white Alices of 100 years ago. It's possible that the present Alice was the result Glen wanted.
 * Jossed. Alice is  And the Will of the Abyss is   Both of them we're the result of   that seems to be the origin of the story of Baskervilles casting sinners into the Abyss.   And as I explained before,   He got what he wanted. Yes, chapter 69 was just about the biggest Info Dump yet. And it works.

Break is going to end up losing his other eye.
Vincent uses his scissors on Break, because he's getting in the way of something, and he thinks that'll keep Break out of the action. He then proceeds to get thoroughly pwned by Break, who was already used to fighting without being able to see. Sure, maybe this particular set of circumstances isn't that likely, but if it happens, it'll be an automatic CMOA.

Leo is going to disappear after this arc.

 * You were sort of right.
 * As for Leo disappearing,

Elliot is going to have a much smaller part after this arc.

 * Retrace LIX makes this... *cough* most likely.

Emily is the one behind everything.
Emily is the one pulling all the other characters' strings to achieve her own ends, which have not been revealed yet.

Leo is the reincarnation of Lacie.
We also obviously have to assume Leo is female. Elliot is either Glen or just in some way reminded her of Glen and she wanted to jog his memory by

Lotti is in some way related to Lacie.
The letters in 'Lacie' can be rearranged to spell 'Alice'. This was originally used in Alice in Wonderland, during the March Hare's tea-party, where the Dormouse tells a story about three girls living down a well called Elsie, Lacie and Tillie. This was a reference to the three girls who Carroll originally told the story to. Elsie was a reference to Lorina Charlotte (note the initials) Liddell, the eldest of the three sisters. Since Lotti was possibly named after her, it could mean that she has some connection to Lacie...

Elliot and Leo are Peppermint Patty and Marcie.
And Break is Snoopy, Gilbert is Charlie Brown, Ada is Sally and Alice is Lucy.

The Will of the Abyss will kidnap Oz.
She came pretty darn close to doing so around the start of the manga, and we know that she had the hots for Jack, whom Oz is the reincarnation of. I think that it would be a really good story arc, or even Grand Finale scenario, for the Baskervilles to attempt to earn the Will of the Abyss's favor in exchange for Oz. Naturally, his friends from Pandora come to rescue him, but only after he's gone through a few rounds of Break the Cutie and Mind Rape.

Jack will at some point be nicknamed
Definitely out there, but it would be a hilarious reference to Lewis Carroll

Jack, Glen, and Lacie were in a Love Triangle relationship.
Still a bit whammed out from Retrace LXV, but I'll try my best to explain this (mostly just to have something, make sense). Jack  Glen and Lacie became closer, distancing Jack from Lacie at the same time. Jack goes Type 2 Yandere for her, which is his Start of Darkness. After planning to kill off Glen, he walks to his room, where he finds out that while he was plotting and planning, Lacie had one child, Alice. Jack switches over to Type 1, and goes after Lacie. Glen, wanting to protect Lacie, uses his chains to send her to the Abyss, where she becomes the Will of the Abyss. He takes Alice and puts her in that tower for her safety. He also manages to get Jack to calm down. They keep their friendship going until Jack snaps again, around the time of the Tragedy of Sablier. At that time, he murders Alice.

With the current reveal in the latest chapter, it threw the whole plotline and what we knew on its head. Such as revealing that. My reasoning now is that what if had been lying to us from the start and that it was instead   that caused the Tragedy in a big gambit to   this could further be supported if perhaps   is related to Glen, such as his daughter or younger sister and had protested their relationship. Similarily, the reason spoiler: Glen's body is in all of the crystals was part of a gambit to seal Jack away due to that yandere nature that was threating to destroy everyone and everything. Oz's father could even be part of a gambit to release Jack and restart it all over again so Jack could have his Dystopia with Lacie. Also, it could be revealed that if all of Glen's body is destroyed, JACK would be released back into the world.

Lacie is Alice and the will of the abyss combined
We arent told much, but from the looks of it the Will of the Abyss and Alice DID at one point seem to share one body, and  Jack DID say that until she was killed, Alice had been human. What if the shock of death had split 'Lacie's soul in two, one becoming our Tsundere Alice and the Mind Fuck Will of the Abyss? Also, in order to set free the Will of the Abyss, Alice and the Will of the Abyss may have to rejoin, which may return the Abyss to its 'former glory' as the Baskervilles seem to try to be planning.
 * Jossed.

The Mock Turtle, Duchess, Twins and Knitting Sheep will figure in sometime
The series has stayed very close to the books, if we're going with the idea that Oz is Alice and Alice is the Red Queen. Every character has fit in somewhere with the original. Therefore, since these five haven't appeared (to my memory) they will be in some way important!
 * We already have the twins in the form of As for the others, we are still in the wait.

Vincent will fully fall in love with Ada
And give up his homicidal tendencies and team up with Oz and Gil. Recently it shows signs that he actually DOES have feelings for her, so it is possible. Either that, or he will have a Heel Face Turn

There are actually several timelines because people keep asking the Will of the Abyss to change past events
We know because of Break's backstory that it is possible to change the past, with unpredictable consequences. After retrace LXV, it's possible that the timeline has been altered twice: the original using, the second the one where  , the third

just a kid who was in the wrong place at the wrong time and happened to be insane.
 * How did it happen that in Chapter 58, though? Also,.
 * Retrace LXX has confirmed that.
 * Retrace LXX has confirmed that.

Think about it.
 * Jossed by Retrace LXX: Oz. He is not Jack's son.

There's a lot more about Break that we don't know
Such as: why wasn't he considered a "Child of Misfortune"? Is it because he's albino? In the world Pandora Hearts is set in, it seems odd that they'd just let that slide. I mean, we know about the  thing, but that's not quite the same as a "Child of Misfortune". Something tells me we'll hear more about this.