Beelzebub/WMG

Beelzebub has siblings who will show up eventually.
It will come to light that baby Beelzebub has siblings, possibly also in contracts. Beelzebub will be the youngest and the only boy.
 * The reason why Beelzebub is the next in line to the throne is due to the Heir Club for Men being in effect.
 * Confirmed, we now know he has at least one older brother, En.
 * And apparently both are being tested to see who deserves the throne.

Miki has a contract. And it's not with a demon.
There has been a reason why baby Beel is so terrified of that guy (ignoring that Miki may just too fucked-up inside for Beelzebub to handle.) and it could be a contract with something opposite to a demon (Good Is Not Nice applies).

Kanzaki is about to take massive levels in Badass.
Right now, that guy's self esteem is probably shot to hell. What happens when that happens? Took a Level In Badass

Oga is gonna lose a fight, and go to Aoi's grandpa to train
Not because he think that it will make him so much stronger, but because that's why happens in anime or video games. An old man trains you and makes you awesome. Furuichi will note the absurdity of this though.

Oga lost a fight to Furuichi when they were kids
This is why in Oga's head, Furuichi is super buff, and transforms into a Super Robot. Also, Rule of Funny would dictate that one of Oga's only losses would be to one of the weakest characters.
 * And then he fights him again with his demon powers and kicks his ass.

there will appear an angel to "Stop" beel's "evil intentions"
If the demons exist, then the angels as well...and with all the chaos oga has done, it difficult that they don't have discover it.

Hilda will die
Maybe later she return, but she will sacrifice to protect oga, and probably will made some kind of "contract" with Kunieda.

Saotome was En's contractor
Their relationship will be a plot point, and may either cause a Heel Face Turn regarding the destory humanity thing, or make it worse, depending on what kind of terms they parted on.

Furuichi has becom a real "General" of an army of demons without knowing

 * Where the hell appears his title of General? yeah, the two that call him it are Alaindelon and that student of saint ishiyama but where it come from? maybe, because being so close to Oga, as demons would think, he qualifies as a high rank soldier of Beel...and Alaindelon would spread the rumor unintentionally, that's also explain why alaindelon's daughter call's him "Furuichi-sama"

Oga will have a Papa Wolf-esque moment
En or some other person will very likely (And beleive me, It isn't unlikely) hurt Beel. Badly. Oga will not take well to this.

Oga's parents were delinquents
That's why Oga is so evil and strong. Because they, unintentionally, taught him to be a strong fighter. The mother could be the red tails leader when she was young, wouldn't be hilarious see the red tails expression at The Reveal?
 * Quite possible. The parents know he's a delinquent yet don't do anything to change that. And a recent chapter revealed (in passing) that, when Kunieda was at Oga's shortly after the Behemoth soldiers attacked, Oga's older sister was the Red Tails' leader before Kunieda! Could they be a family of badasses?

The four member's of the Touhoushinki will make a contract with some demon
Confirmed as seen in the latest chapters. Not only she defeated one of the Pillars of Behemoth's Army with relative ease, but is also training with Oga to acquire skills that are unattaiable to the rest of the Ishiyama crew.
 * Kanzaki and himegawa are starting to have more and more protagonism. it's only time to have them discover the truth, and then, they will ask saotome how adquire a contract to go out from their Badass Decay.
 * Kunieda knows the truth and it's probably she wish to do something more, because she wasn't pretty useful in the last battle...
 * Toujou...he is an idiot, but he has saw with his eyes what happened to the school and Saotome will train him. it's just time to see him discard his Idiot Ball and discover the truth.

When Oga has his own child after the events of this series he will be the best Dad ever

 * He will have had so much practice raising a demon king who can shock him if he messes up, and with parenting, like everything, practice makes perfect.

The ending will be one massive Tear Jerker

 * Think about it. Oga and Beelzebub are growing more attached to each other, and seem to genuinely care about each other. Eventually, Beel (and likely Hilda too) may have to permanently part ways with Oga in order to take up the mantle as the Demon King, to many Manly Tears. What might make it more effective is if Oga becomes a better father than the Demon King ever could be.

Furuichi will make a contract with En.

 * And as a Shout-Out to Black Butler, the demon's mark will be placed on his right eye and he will wear an eye patch over it from that point on. However, unlike Oga, he'll understandably be more concerned for En's, and his own, safety (since En's older and logically more powerful than his younger brother, plus he has a tendency to piss other people off, which would lead to some difficulties).

The series will become a full-fledged Fighting Series.

 * This troper hopes he's not the only one who sees this. While it'll probably be a more light-hearted, comical one akin to Dragon Ball, the series is getting more and more serious and the fights are getting bloodier, a prime example being the fights against the demons. Not to mention the constant implications of serious battle, such as references to Behemoth and the appearance of Athrun as early as the Summer in Hell arc. AND the Training From Hell for Oga to master Beel's true power (which funnily enough, is a send up of Dragon Ball's own training arc).

Baby Beel is the Demon King's favourite son/the son that Demon King wants on the throne.

 * He sent Beel down to destroy the earth first and gave him the more superior wet nurse. A sign of possible favouritism? Plus this might be the reason why the Pillar Squad want to get rid of Beel and Oga - so En will have a better chance of getting the throne.
 * The fact that En got his own squad might invert or subvert this, depending on your opinion. Maybe the demon king realized how biased he was and gave him the squad to balance it out? That gives immense implications to Hilda and Alain Delon's power.

Mr.Saotome is Oga from an alternate timeline/alternate universe, possibly trying to Set Right What Once Went Wrong.

 * For god's sake, They practically mirror each other. For the alternate universe, it would explain why he had the demon mark on his shoulder rather than his hand. On the other hand, the timeline plot is much more interesting. It would imply something really serious happened, and is going to happen, that was so terrifying that he had to go back in time to try in fix it. It would possibly also explain why he's older and less forthcoming-either he went through that many attempts, or (heaven forbid) He actually took time to plan out ways to execute what he was going to do. (The latter is unlikely, even with Break the Cutie moments, given Oga's status.
 * Some support, In Chapter 137, Toujou mentions Oga is getting more and more similar to Saotome.

When the anime is licensed in English, Furuichi's voice actor will be Todd Haberkorn, the English voice of Kimihiro Watanuki

 * Can you think of any other person who could pull off the Only Sane Man schtick as well as he does?

Related to the first WMG, Beelzebub has 6 siblings
They're the Seven Princes of Hell from Christian Demonology! This almost seems obvious. The only question is what "En" is short for.
 * 1) Lucifer, demon of Pride
 * 2) Satan, demon of Wrath
 * 3) Mammon, demon of Greed
 * 4) Asmodeus, demon of Lust
 * 5) Belphegor, demon of Sloth
 * 6) Leviathan, demon of Envy
 * 7) Beelzebub, demon of Gluttony.
 * I could say that he is the demon of Envy. His full name could be En "Leviathan" etc.

Humans can be turned into demons without a contract, and at least one character will have this happen to them
If you're wondering how, it's possible that if you pump enough demon energy into a person, they'll turn into a demon. It could also be a chance for gratuitous Fan Disservice, Body Horror, and Nightmare Fuel. Now the only question is who the lucky guy will be. (My vote's on either Kanzaki or Hanazawa.)

The Great Demon King's Arrival on Earth
When it does happen (when he has free time on his schedule), he'll be the Knight of Cerebus that ushers the series to a new direction. His behaviour might make it seem other wise, but he has to be the Great Demon King for some reason. It's likely that he is a Cloudcuckoolander, but that just makes him more of a ticking time bomb in that regard.
 * Psychopathic Manchild, Maybe? It could explain His Behavior Toward his sons. Better yet, instead of Straight up Cerebus Syndrome, the series will head into a mix of it and pitch Black Humor.
 * In the anime he's makes an appearance.

Naga is a Bifauxnen

 * It seems like a bit of a stretch, especially since we've seen at least two women in the army, but it doesn't have to be for the obvious reasons. Rule of Funny could dictate that it was something stupid, like not have the right cup size.

General Furuichi will actually become a General
...Or at least a Colonel. Because who would expect the Butt Monkey of all people to be a Military Genius?! Also, there's the fact that already has some serious tactical experience.

When Hilda finally regains her memories.

 * The first thing she is going to do is punching someone in the face.Hard. Maybe Oga.
 * Or Creepichi. Actually, DEFINITELY Creepichi.

Hilda remembers everything that happened during the amnesia period.
She's just too embarrassed to admit it.

Furuichi is going to die.

 * It would be a perfect way to set off Cerebus Syndrome-simply Deconstruct the Butt Monkey by finally having him killed during one of his humiliations. To add insult to the injury, nobody will even remember him.