Ah! My Goddess/WMG

Keiichi will receive his own spirit
maybe not an angel or a devil, but something else. But it was hinted that he still is able to have a spirit living in him.

The Niflheim Arc will end with Urd becoming the new Daimaikaichou
This doesn't really fit with her character development very well but it make for a brilliant Curb Stomp Battle.

Hagall is going to have a Cry Cute moment when Hild decides not to desintegrate her, dispite her rebellion
And it will be absolutely adorable!

Belldandy is mentally retarded.
Think about it. She's completely oblivious to the trouble she causes Keiichi. Her entire job involves working with humans, but she has almost no idea of what humans are like. She is completely ignorant of the ways of humans and of life on Earth. She is unable to determine the motives of others, even when they're obvious. She never considers that someone might have ulterior motives or might be lying to her, even if they have before. She willingly invites a self-declared enemy (who has tried to seal her before and almost destroyed the world) into her house for lunch. Heaven knows this, which is why her contract with Keiichi was approved even though it is extremely unusual (Peorth laughed at the very thought of such a contract receiving approval). That is also why she was given a little more time on Earth even when the contract was destroyed. That's why the Almighty One was willing to look the other way when his subordinates were breaking the rules to try and restore it. Heaven simply wants her gone.

Belldandy's perceptions are being clouded.
It's a test of both her and Keiichi; him for not giving up, and her for breaking past the clouding. Kami-sama is making sure it will work out before such an unusual pairing is allowed.

Belldandy getting really mad would be just as dangerous as Urd thinks.
Note that she can overwhelm an individual by use of a Compelling Voice, just by being moderately peeved. When she "believed" that Skuld built "Texas Murder" to "help with the gardening", the thought process might have been, "I'm going to assume that's a gardening tool, because otherwise, I'd be obliged to blast Skuld off the face of the Earth♥!". If she's as powerful as Urd seems to think, she has the same world of cardboard problems as Superman, and puts on the oblivious persona as a guard.
 * Supported by the events of the animated series; when Belldandy's emotions veer to simply subconscious jealousy, it's sufficient to knock out an entire hallway of light in publics.

Takano has mysterious powers of clairvoyance.
Takano, from her appearance in vol. 23 to her departure in vol. 24 of the manga, repeatedly demonstrates knowledge of events that she was not present for, which she variously attempts to pass off as motherly intuition or eavesdropping, but there is one instance that cannot be thus explained - Takano brings up Chihiro's envelope, given to Keima While there is a period of time in which Takano is not "on-camera," it's heavily implied that she spends the entirety of this time playing Mah Jong against Urd, Skuld, and Peorth, none of whom knew about the events in question. Keima and Keiichi were racing against each other, while Belldandy was performing in the role of hostess (and also giving Keiichi a tiny bit of help). Megumi was helping Bell with the refreshments during this period, and while she was present for the giving of the envelope and could have passed it along, this is never shown or referred to.

Belldandy's father is Balder.
This is not my WMG, but I think it was mentioned on rec.arts.anime.misc once upon a time.
 * Jossed; when he shows up (in Gate's body), he introduces himself as Tyr -- and Heaven's CEO, so he is the Almighty, after all. Note that The Other Wiki indicates Tyr was originally considered to be chief of the gods until Odin edged him out.

Belldandy planned Keiichi's wish.
It's not clear how she knew that he was going to wish being with her (she only explicitly implies wish-earners are not the sort to wish maliciously) but when she teleported to him in the original OAV, we see an image of the temple that they eventually live. She also knew precise directions to the Temple when they hit the road. She somehow planned his wish and their getting kicked out of his dorm.
 * That Belldandy has a sneaky Xanatos Gambit side isn't too surprising, given her old teacher was the OMG version of Lucifer.
 * That was all Loki's plan, he wanted to cause Ragnarok, but seems like he failed. Again.
 * She may not have planned it, but during the Niflheim arc she admits that she'd been watching him for some time prior to the wish,

Fairy Morgan Le Fay thinks that Keiichi is the reincarnation of her lost lover.
(This is in the movie.) And that's why she makes such a determined effort to steal Keiichi from Belldandy. It isn't just that he's the geeky guy in the middle of the harem show so every cute girl will fall for him. It's much deeper than that: she's trying for a second chance with him. It isn't until she sees Keiichi and Belldandy walk through the arch that she finally gives up on getting him back. That Keiichi might actually BE isn't impossible; people can fail the test of the eternity gate.

Keiichi IS Morgan's long lost lover
And they failed the test of the eternity gate.

Keiichi's Wish isn't specific to Belldandy
The wish "I'd like a Goddess like you to stay by side forever." doesn't specify Belldandy. Which allows for Urd and Skuld to stay on Earth. And allows Peorth, Lind and the others to visit. Belldandy can leave anytime she wants to as long as Urd, Skuld or any of the other Goddess' stay on Earth with him.
 * Not to mention that Bell doesn't seem like the sort to bind some random unwilling goddess to the wish's terms. She probably couldn't anyway, since it would be a mild form of slavery, and is probably as illegal in Heaven as it is in Japan.
 * This has proven to be false on multiple occasion by the statement that Keiichi's wish was Keiichi asking Belldandy to say with him forever.
 * At the very end of the series, she tells him that

Bell and Keiichi have already slept together.
Perhaps several times now. We the readers just haven't seen it, of course.
 * Actually, it's fairly obvious that they've slept together. The question is whether or not they've had sex.

Keiichi is unknowingly something other than a normal human -- possibly even a god himself.
In the OVA's flashback to their childhood meeting, Belldandy remarked that no human should've been able to see her arrival. Either she overestimated the strength of her Somebody Else's Problem field (she was a child, after all), or this is canon evidence that K1's perceptions are beyond human.

Keiichi's mother is something supernatural
Not necessarily a goddess but definitely something. This would explain why she doesn't show her age at all, the way that she and Keima quickly identified what the goddesses where and all of the unusual traits that Keiichi has.
 * Also Lind might be in on it.

Urd is Hild's doublet
This leaves us at one of two outcomes, and possibly both: either Hild is old enough that she proceeded the double system (and maybe proposed it herself), or before Urd, something, perhaps very frightening, happened to Hild's last doublet.
 * I seem to remember that Hild mentioned agreeing to the doublet system for Urd's sake. Also the entire point of the doublet system is that Hild would die with her doublet.

Because of the wish, technically, Keiichi is a god in a human body
Even if the wish itself was canceled, there is something weird in it: the wish is that Belldandy and Keiichi be always together. Now, it was canceled, but the wish itself said they had to be always together. At the time, no one knew the wish would be canceled, and as wishes in general are granted instantly, it's for sure that they solved the death thing in that very moment ... but no one realized it yet.

Even without the contract, Belldandy's goddess status and powers are bound to Keiichi
Simple: She loves Keiichi with all of her being. It is heavily implied that Love is the greatest source of a Goddess' powers. Furthermore, as a prerequisite to being a first-class goddess, Belldandy must never, ever tell a lie. If she decided to nullify the contract and return to heaven, she would be betraying her feelings, technically lying to herself in the process. She would remain a goddess, but will lose her first class status, with her immense powers and wish-granting occupation along with it.

Kami-sama is a Gnostic-styled Demiurge.
He runs the universe, but like many gods would not need to be its creator. Thus he may be the Goddess' actual father (a common WMG in Western fan circles, since the translated comics have them calling him "Father"); if this is true, then he and Hela were an item once upon a time.
 * Seriously and he's kind of an asshole too. His punishment for Urd sure seemed harsh to me.
 * Odin of Norse mythology didn't create the universe either. And he is often called "the All-father". In the myths he comes off a lot harsher then Kami-sama.
 * The "Father" thing is probably just a western influence. It's what you call a bishop (Presbyterian, so I wouldn't know), and it's part of the Holy Trinity-the Father, the Son, and the Holy Ghost.

The Verse runs on Equivalent Exchange
Clues can be seen in the Doublet System, where killing a goddess or demon has the potential of killing the corresponding number on the other side (though This Troper personally questions the effectiveness of it unless EVERY goddess has a Doublet) and the chapter in which Keiichi was the victim of a "automatic luck-balancing system". A- Might kill one of their own. B- Might kill themselves C- Kill a friend or family member. And do you think that, lets say, Urd would try to kill a demon if it meant she might also kill one of her sisters?
 * Keep in mind that those who are in a doublet don't know who their partner is. So if they kill someone on the other side they-
 * Likewise Senbei the genie's personal philosophy is the idea of a finite source of happiness that can be moved around for malicious purposes. This is treated somewhat comedically, but may explain why Keiichi never complains too loudly about his life; he knows it's his generous karma being balanced out.

Reincarnation Exists, and this isn't the first time Belldandy has granted this kind of wish.
The question of what happens after mortals die has never been addressed in canon, and rarely touched in fics, though for some authors, it seems that Keiichi and Bell will be eternally separated when he dies, because the Heaven that Belldandy comes from is different from any positive afterlife. If this is true, you'd think Belldandy would have let slip something by now about how they only have about a hundred years at most together.

Therefore, it's quiet possible that souls get Reincarnated, and Belldandy only works at the Goddess Hotline because she's waiting out the centuries until the soul of her lover gets reborn and calls her back, and restart their whole relationship at the courtship stage. She never knows who that person will be, but that soul recognizes Belldandy each time, and thus makes the same wish. What we're watching now is only the latest cycle.

Urd knows of this, and since she wants Belldandy and her lover to have as much happiness as they can until the end of the cycle, she's always pushing the two to hurry up. As for Skuld, she either doesn't know because she was either too young to remember the last time, or wasn't born yet, or she does remember the last cycle, as well as how sad her Big Sis was, and wants to spare her the pain.

there is a third divine group besides the gods and demons
I think in Djins and/or Shinigamis. Djins because...well, it was recently explained that the gods and demons fulfill humans wishes in exchange of their energy(or something ike that) but without explaining why. The demons can qualify as Jerkass Genie but i really can't see them as tricksters who play with the person's word and make him suffer and/or die. The shinigamis is because the death thing wa snever explained so far, so, as the valkiryies are an elite force more than reapers, as they are described in the myths, i think in shinigamis(or reapers) as the ones designated to watch the dead ones.

Kami-Sama and Hild are aliens who consumes emotional energies emitted by humans as means of life-support
...and goddesses and demons are cybernetically-augmented humans they created to provide and maintain a constant amount of energy flow from earth.

Here's the theory unfolded:

Long, long ago, in a galaxy far, far away, Kami-sama and Hild's homeworld was destroyed, leaving them as one of the last surviving members of their species. They barely escaped death and travlled across the universe to find a suitable planet for living. Eventually they found earth, a planet where intellectual species emit emotional energies strong enough to sustain them. Together they manipulated human society via means of religion, politics and more in order to draw out energies, and the eras went on.

Unfortunately, as time passed by, Hild became increasingly greedier at consuming energies. She decided that negative energies are much easier to draw out via the means of chaos, misfortune and death. Hungry for power, she abducted hapless humans and cybernetically-enhanced them to become her harbringer of misfortune, then set them free to spread chaos on earth. Them are the first demons.

Angry at Hild's doings, Kami-sama created his own minions - goddesses via similar means as an attempt to thwart her misconduts. Naturally, it broke out into a war, destroying much of earth's energy systems and living the clueless humans confused and frightened.

The war took its toll, realising that this will ruin the world for both of them, Hild and Kami-sama came to a truce. And from there on they parted their ways, assigning the duty to keep the energies balanced, which was to send goddesses/demons to help people in need.

Therefore, any of the "magic" we see the goddesses/demons perform are merely bio-engineered alien technologies at play, and "heaven"/"underworld" are alien spacecrafts which act as life-support for Kami-sama/Hild and mainframes/serve site to support the goddesses/demons.

This would also lead to the next theory:

Keichii emits an unusually large amount of emotional energy for a single human being, which is why all this is happening to him.
Assuming the above theory is true, then it would explain why Keichii gets so much attention from both sides. And even though the goddesses are with him, misfortune still happens to him on a daily basis (though admittably on a much smaller scale). These are Kami-sama and Hild's means to keep his energies balanced, which, although reduces his misfortune, would probably means that he'll never make further progress with Belldandy as it is, because Kami-sama and Hild are actively sabotaging any romance between them.

The Norn sisters were the three Minami sisters.
Belldandy was Haruka, Urd was Kana, and Skuld was Chiaki. Before being granted their goddess licenses they had to spend some time living among mortals to become more familiar with their society. However, one of Yggdrasil's many glitches incorrectly assigned three Norse deities to Japan. With Belldandy gaining great proficiency in the Japanese language and culture, she was eventually assigned to the East Asian division of the Goddess Helpline...and the rest is history. (When they returned to Earth to live with Keiichi, a similar glitch flipflopped the "ages" of Belldandy's and Urd's mortal bodies.)