Blue Exorcist/WMG

Mephisto Pheles has Minored in Asskicking and will have to get dangerous at some point in the story.
Because, really, the guy was able to hold back both Rin and Amaimon at the same time without looking strained at all. Sure, Amaimon punched him in the face to get him to let go, but it probably just caught Mephisto off-guard. He practically laughed it off afterwards, calmly captured Amaimon, and then went and did this as if it were just a walk in the park for him. Keep in mind that, before he got punched in the face by Amaimon, Mephisto was trying to explain that there was an "overwhelming difference in" Amaimon and Rin's abilities. Presumably, he meant that Rin was potentially more powerful than Amaimon. Amaimon. The badass whose mere poke can break bone. He is said to be pretty much impossible for any exorcist to beat. Rin is supposedly stronger than this guy, when berserk. Mephisto was able to drag Rin, hold his arm in place, and sheath the Koumaken sword while holding casual conversation. Let's just say that, if Mephisto doesn't get the chance to really show off his power, it would be severely disappointing.

Todou is Yukio from the future.
I've had this theory ever since chapter 16 came out, but I was lazy so I didn't put it up. Toudo is Yukio from the future. Yukio seems like he doesn't have any demon powers yet, right? And Satan doesn't seem that focused on him unlike the way he is on Rin. So maybe it's all just one Xanatos Gambit by Satan Yukio's powers to develop later on than Rin's and become a demon. And plus, the way Toudo talked about not being able to be like his brother and father seems a lot like foreshadowing Yukio's eventual transformation. Plus, Shura even says that he's at risk of becoming a demon. Plus, after Todou
 * A funnily related (yet probably unintentional) fact - when Fukuyama Jun, Yukio's seiyuu was asked to act out one of the lines during Yukio and Todou's confrontation(when Yukio says "I'm different from you!") in a series of interview videos posted on nicovideo, he was oddly insistent upon playing the part of Todou as well - only speaking Yukio's one line after much egging from Okamoto Nobuhiko.

Mephisto is genuinely on the side of the exorcists and wants to kill Satan
Fujimoto clearly trusted him completely, having intended to send Rin to him for some time. Add that to the fact that Mephisto being evil just seems too easy. I mean, the demon that betrayed hell didn't really betray it? Come on, that kinda thing is just done to death.
 * This troper honestly can't see him going in any other direction, especially since he has already stated, in a private moment where he would have no need to lie about it, that he plans to never return to Gehenna. Assiah is too much fun for him, in many ways. He seems to genuinely enjoy Assiah things such as Moe and candy and Otakudom in general. Why would he ever want his father to destroy all of that?
 * this could also indicate that Mephisto getting Amaimon to learn about Assiah (starting with Japan) could be a move to get Amaimon on his side.

Satan has dork genes
Just look at Rin, Mephisto, and Amaimon; they all have traits that could be called Adorkable. Then there's Satan while he's possessing Shiro Fujimoto and how he acts then. It's obvious, Satan has dork genes and Rin, Mephisto, and Amaimon inherited them.
 * This troper can absolutely believe that. Even Yukio has dorky aspects to him.
 * Satan...dorky. You, my friend, have an odd idea of dorky. Although his laugh came off to this troper as very generic villainish.
 * This troper first came up with this theory before the anime started. You can conceivably read Satan's lines from the manga in a dorky!evil headvoice and have it work pretty well. And it also has to do with the line choice; just because someone is an evil villain doesn't mean they can't have dorky factors to them. And it's highly likely a villain with dorky leanings would go for an overblown generic villain-type actions.

Noriko Paku is at least partially Korean.
The kanji for Paku is 朴, which is also the kanji used by Romi Park in her last name. Romi Park is of Korean descent and her last name can be read as Paku, but is more accurately Park. So maybe Noriko is the same and her name is more accurately Noriko Park.

Izumo is also a Half-Breed
It would explain some things...
 * Or perhaps all summoners have at least some demon genes in them. Most of the people in the series are summoners seem to have a trait that is off. Even Shiemi's naturally blond hair could be a sign of her having a demonic lineage because most of the other character with non-dark colored hair have been shown to use hair dye.

Rin will take over Fujimoto's church at the end of the series
The only evidence here is Fujimoto randomly asking Rin to take over the church if he's gone. Then Rin says "Hell no!" Considering all the other times this has happened to the hero (in other series)... Hm... wild mass irony?

Neither Rin nor Bon will kill Satan
Yukio will deliver the final blow.
 * (Another Troper here) After a Heel Face Turn, since many speculate (and its been hinted) that Yukio will turn evil at some point.

Rin will open the Gehenna Gate
Even though Satan admitted that only he can do it, Rin inherited all of Satan's powers. How else is he going to face the old man in the end?

Yukio will have a Face Heel Turn
This had to be put eventually. There have been quite a few signs and hints pointing to this direction. Shura pointing out that he's at risk of becoming a demon, and Toudou's speech to him, and their slight similarities being two that stick out to this troper.

Mephisto will use Rin to defeat Satan and take his place as king of Gehenna
Yeah. Mephisto has made it clear that he wants to raise Rin to become a weapon against Satan, but it's easy to tell that he's not exactly "good." It confused me for a while, before it hit me: Mephisto does want Satan out of the picture, but not because Satan is evil, but because he wants to rule Gehenna himself. Mephisto either believes that he will never be able to defeat Satan and Rin will, or he's just a Magnificent Bastard and doesn't want to sully his hands. If you think about it, it makes sense and a lot of things fall into place that way.
 * Even a manga translator guessed that this would happen. It seems to be a fairly popular theory and it's further supported by the fact that Mephisto is the eldest son of Satan that we've seen so far. He'd probably be next in line, since his father still seems to think that Mephisto is loyal to him. If there's one thing that's clear, it's that Mephisto wants to take down Satan. Why that is, we'll have to find out. The one problem with the the theory of Mephisto wanting to take over Gehenna though, is that he himself has said in an aside (with no one around for him to lie to) that he never wants to go back to Gehenna and that Assiah is too much fun. To rule Gehenna, wouldn't he have to return to it?
 * Then maybe he wants to destroy Gehenna and be done with it. And this may seem all fine and dandy at first, but maybe it'll be something like Gehenna is a necessary evil. Although Satan is bad, destroying his dimension would cause some crap on Earth. Mephisto, blinded by the immense power he's gained by having Rin defeat their Father, ignores this fact and goes on a rampage.
 * Sounds a bit Out of Character for Mephisto, being a Magnificent Bastard, to go on any sort of rampage. But it's still plausible. Would also be an interesting twist for the story to take since it would make Mephisto the true Big Bad. We'll just have to wait and see.
 * I've always seen Mephisto as someone who would rather be The Man Behind the Man. he likes pulling the strings, but he also likes a level of freedom to act as needed, more like the type who would want to replace Satan with someone easier to control or with compatible goals than actually take over himself.
 * Now that sounds more plausible. Still leaves one wondering just what his true goals are though. Amusement seems likely.

Rin and Yukio's Mother will become important in some way later on in the series and Father Fujimoto will return just long enough for Rin to properly finish things off with him
Okay so I admit the first one is just a gut feeling (albeit a really strong one)but the second one seems likely as (even though Fujimoto's death was merited by Death By Origin Story and Too Cool to Live)... Rin would've wanted him to see how far he managed to come. Or something like that.

Mephisto Pheles has been manipulating Todou Saburota all along.
In fact, the awakening of the Impure King is part of Mephisto's plan to force Rin to use his flames to defeat the Sealed Evil in A Can. Mephisto may or may have not personally told Todou exactly what to do in the first place. , as planned by Mephisto, will only increase Rin's power against the Impure King. With that power, Rin will be able to destroy the monster, which will greatly impress the rest of the Order. As such, they will give Rin and Shura the necessary time for Rin to master his flames enough to make him a powerful exorcist to fight against Satan. Todou's recent will be of no consequence to Mephisto, who will simply have him eliminated when the time is right - either personally or by ordering someone else to do the deed. Possibly Amaimon.

This is backed up by some evidence in that Mephisto Pheles has been noted as being one of the few people who knew where both Demon Eyes were. Also, Mephisto seemed just a little too cheerful on this page. He didn't really seem like he had "lost" at all. In fact, he looked like everything was going exactly as he expected it to. Plus, if Mephisto really is the Guile Hero (or Magnificent Bastard?) he's been shaping up as, there's no way he'd let his enemies trick him so easily. If this WMG is right, he's been planning everything since the theft of the genuine "Left Eye." Todou, Shura, and Yukio have all been his pawns, which Shura herself has even implied here. Let's just hope he's a Guile Hero.

Fujimoto was actually Satan all the time
all the times he appeared, it was actually a decoy.

In the Anime, Amaimon didn't die, he just had his body in Assiah Destroyed
that Hamster...
 * Confirmed as of episode 23.

Yukio is next in line to be possessed.
Satan targeted Fujimoto (the person closest to Rin) for fifteen years - who's to say he isn't going to go after Yukio, who is probably the person closest to Rin now? It would work with the 'Yukio will turn to the dark side' theory (albeit, it'll be involuntary). Alternatively, he'll go after Shiemi.
 * Confirmed as of episode 24 of the anime. The manga, however, still has yet to put in any hints about Yukio having Satan's powers at all.

Takara's pink bunny puppet is a parasitic demon or Takara himself is demonic.
We know the other random "why are they even there" character turned out to be Shura, who was working for the exorcists, so why wouldn't the demons have put in their own mole to make sure everything is going okay and try to keep an eye out to make sure Mephisto isn't going to go off on his own set of plans which could clash with Satan's. We also know he's strong enough to have finished the lantern trial on his own and while Izumo did it on her own as well she had the help of her two familiars, while Takara doesn't have access to familiars. I'm leaning toward the parasitic demon theory more since that may be something easier to get past the radar than him being a demon.
 * I'm positively agreeing with you on this, at least in the Anime, something is really strange with him, he was seen on the same place Mephisto and Amaimon were, watching the events of Ernst's attack in Gehenna, reciting verses about Abbadon in a Creepy Monotone as Ernst is enveloped by the Locusts and thrown into the Gehenna Gate, he may even be Abbadon himself!

Just as Amaimon is the "Earth King" and certain demons are described as his kin, there are demons who are 'kin' to Rin.
Both are Sons of Satan, after all, and this could include Mephisto who simply abandoned his. Rin's kin might intend to drag him to Gehenna or are simply waiting for him to grow stronger before directly appearing to him. Whether their loyalty is to Satan or Rin first may be an issue of contention within their numbers already, now that they have a 'King'.

Rin is Satan.
Pure crack theory here, but the thought came to me the other day... Okay, so we know that Rin has inherited Satan's powers, but imagine what if it was more than just Satan's powers Rin inherited? What if Satan had planned to create a vessel for him to exist in Assiah but the process resulted in him being born in Rin's body but no memory of before? After all, how much indication do we have that in the manga canon Satan has been directly involved with most of the events occurring after Rin and Yukio were born? Yes, I know we have Satan showing up in the opening chapter... but the theory I have deals with that, too. If this crack theory was correct the fact that Satan would essentially be missing would probably be something that would get covered up by the few that know, either out of loyalty or to manipulate the situation to their benefit (or both, after all, they are demons).

Now, it's implied that Satan did have other kids as some of what Amaimon says indicates he is one, and possible all of the Princes of Hell may be as well (that's assuming he's not speaking symbolically and that the title of Prince of Hell has to do with parentage instead of power), so why wouldn't any of Satan's other kids have any of his power while one of the kids he had with a human woman wind up with it? Everyone assumes that Rin and Yukio are fraternal twins instead of identical ones, but what if Satan had a plan to create a body that could allow him to exist in Assiah by getting a human woman pregnant and in the process of development the plan was to create identical twins but manipulate it so one was mostly human and the other being mostly demon, with that one being the one that would become his vessel.

Plans like that tend to be a bit more complicated than they seem however, and somehow Rin didn't wind up with any of the memories of his existence as Satan. Perhaps something about being shoved into the existence of a fetus had to do with it or the sealing of the blue flames after being born. Maybe it was even some other odd factor or a combination of factors, but somehow that happened. It's almost certain that Mephisto had a hand in sealing Rin's blue flames and we know he's a tricky, manipulative one, so what if he guessed what Satan's plan was as well. He's the type who would like to create a figurehead leader who he could manipulate so maybe he had an active hand in those memories not transferring, intentionally sealing them away in the Kurikara along with the blue flames.

And as for the whole incident where Satan possessed Shiro Fujimoto, perhaps the whole thing had as much to do with the seal on the sword weakening as much as it had and Rin's state of mind having found out he wasn't fully human as Shiro's reaction to Rin's words and the power and memories sealed in the Kurikara possessed him in their attempt to make the original plan work and awaken Rin/himself properly. And perhaps something in how the power was sealed in the Kurikara made Rin releasing it himself cause only the physical awakening without any of the memories awakening as well. Again, this could be by Mephisto's design, betting on the humanity he'd gained living without any knowledge of his past existence would cause him to draw the sword himself rather than have his awakening be forced by another power.

And as for Rin, there's the question on his personality in the case he was unknowingly Satan. Is his personality simply the result of living 15 years as a human or is there some of his original personality as Satan involved as well. If you go with the Satan as a fallen angel belief you could guess perhaps some of Rin's personality comes from how Satan was before his pride got the better of him and he was punished for it. And the question begs to be asked, if he ever gets his memories as Satan back which personality would be the most dominant one, the one of Satan, subduing the personality of Rin, or would his current personality be enough to affect his old one for the better.

Now there's the multiple possibilities in Mephisto's role in the whole thing. Did he know Satan's plans from the start? If so did Satan let him know or did he find out enough to make him guess what was going on. Does he want to create a Satan 2.0 to manipulate to his own whims, does he want to work it out so Satan is going to owe him one, or does he like Assiah enough to want to set it up so Gehenna is lead by someone who would be better at keeping demons in line (while being able to manipulate them to his own whims).

Shura is Kamina and Yoko's daughter
'Nuff said.
 * And Kyouko Sakura is her daughter. Somehow.

Anime!Satan is Karkat Vantas.
He has enough pent-up anger and cursing to be him.

Mephisto's mother is Fem!Italy
His ahoge makes it obvious.