Fruits Basket/WMG

Hanajima knew about the Sohma curse the whole time.
Since she is psychic, it is possible. Also, it was somewhat hinted at the end of the manga.

The cat's grotesque true form is a result of a curse from god & not anger at the curse.
This troper got this guess based on the "true" origin of the zodiac curse. Summary: Basically god was lonely until he found companionship with a cat which gave him the idea of inviting other animals for a banquet whom became the members of the zodiac. Every night god & the animals partied until the cat fell on the verge of death. God then bound the cat, God & other animals in a circle of reincarnation which the other animals happily agreed to not wanting to be separated, however the cat disagreed stating that all things had to end. The other animals & God instead of musing upon what the cat said considered it an act of rejection. In anger at this supposedly betrayal, God cursed the cat to be born in its hideous state not only as "punishment" but to ensure that it would be unable to bond with other people, thus ensuring its complete devotion & making it that the cat would never want to leave God. Support for this would come from Kyo & Akito's relationship. Akito insists on preventing Kyo from forming relations with anyone insisting that only he will understand him.

Tohru is secretly a mass murderer.
She's always so... happy. shes suffered so much and she still smiles, but what if its a slasher smile? What if she snapped years ago? She's nice to the public, yeah, but who can tell? or, in a reversal, she's secretly maniacally depressed.
 * Oh, no. She's nothing so mundane. She's a demon. On dark nights when you're at your most depressed and vulnerable, she'll come to you, whispering reassurances. If you respond, if you let her comfort you, she tears your very soul away from your body and devours it. But her smile never changes, her words never cease, even when you realize they are worse than hollow.

Tohru represents Canada.
She's excessively polite, attempts to keep the peace among her quarreling friends, and her colors (as seen in the swimsuit chapter) are red and white.
 * I would like to expand this theory by suggesting that Fruits Basket takes place in the same universe as Axis Powers Hetalia
 * If that's true, then I'm willing to bet that Momiji represents all three Axis Powers - He's from Germany, lives in Japan, and acts like Italy. How it's possible to be all three though, I don't know.

Momiji is Obfuscating Stupidity
He acts like a child and pretends to cry whenever Kyo hits him hoping to catch him off gaurd and hit him back. The only reason why he hasn't yet is because he didn't want to do it in front of Tohru.

The "god spirit" possessing Akito Sohma is Aizen Sousuke.
They describe themselves as "possessed" by spirits, not that they "are" the spirits. Akito is sure she's God, which is Aizen's general claim. Akito dresses in men's clothes because Aizen is sick of wearing frilly womens' kimonos.

The Sohma family is inbred.
It explains all of the mental disorders in the family. Ren is Obsessive-Compulsive and abusive, Akito is sociopathic and sadistic, Yuki has insecurities, Haru has multiple personalities, Kisa's a mute, Momiji has A.D.H.D., Hatori is bipolar, Hiro is jaded, Ritsu also has insecurities, Kureno has attachment issues, Shigure is a pervert (which could be stretched to be a sex addiction), Kagura is extraordinarily Bipolar, and Kyo has anger issues.
 * Except Kisa's mute-ness came from outside sources, but I am willing to stretch that into a reaction of GAD-turned-w/Agoraphobia (She ran away from home to hide from people) And it ties into the large amount of other Anxiety issues and issues which have a co-morbidity with anxiety issues.
 * Personally, I think Shigure is also a sociopath[I'm using the manga as my canon here], but bipolar people have manic states and depressions, none of which Hatori or Kagura have. And note that both Ren and Akira have vague illnesses, and Akito often suffers from vague illnesses. Yuki also suffered from health problems. I don't think Ren is OCD, due to her lack of compulsions. She just is obsessed unhealthily with her dead husband.
 * I'll grant that Hatori never has a manic state (he seems like he has depression in spades, though), but Kagura clearly has both states - while people focus on her manic Yandere moments, she gets rather depressed at times (particularly later on, as she muses about how Kyo loves Tohru). Also, given how he acts when nobody can see, I wouldn't say Shigure is a sociopath - he does what he does because he thinks it's necessary (even though he does ultimately  for his own benefit primarily).
 * With regards to Hatori being bipolar, he could always be type 2. It's categorized by having the violent swings of emotions yet not showing it (normally keeping a stoic or emotionless facade) from my personal experience, but its clinical definition is that it's just like regular bipolar disorder, just that the manic state never reaches full-on mania.
 * But Ren is just a Sohma by marriage, not by blood. She was a maid.

Yuki's mother wanted him to be the cat.
Yuki's mother is the only zodiac mother we know of who had another child with the perfect knowledge that it was possible that child might be born cursed. Since she had already given birth to the cursed Ayame, it's reasonable to assume she learned a lot about the curse, including the curse of the cat. She could have known that, while the cat himself is a pariah, the family members of the cat are extremely prominent and highly placed within the family (note how Kazuma, the previous cat's grandson, is so revered).

What if she knew that the previous cat had died or was dying? It's possible she may have gotten pregnant with that goal in mind: to be not only the mother of two cursed Sohmas, but the mother of the shunned cat. Although the rat is certainly revered, she never got over the disappointment of not being the mother of the cat, and always resented Yuki for it. It would also add a nice symmetry--we know that Yuki and Kyo wish they were more like the other, and we also know that Kyo's dead mother wished that her son had been the rat. Wouldn't it also make sense for Yuki's mother to wish that her son had been the cat?

Akito actually is a boy.
He was raised to think he was a girl by his mother - who wanted him to be one, hence the cross dressing and 'hiding' his gender from the rest of the Sohma's. Being as obscenely rich as the Sohma family is, Akito's mother could certianly afford to have sex-change surgeries done to him, as well as medicines to help, with bribes to keep the doctors silent.

Well, that, or when he changes into the 'God' - like the others change into their Zodiac forms - he changes into a girl. But being The God, he can transform at will, which messes with his already frail body and mind, perhaps explaining his/her mysterious illnesses.

The Sohma family is really a cult.
Throughout the whole series, the readers learn that the Sohma family is twisted. Going through the cult checklist, they appear to have many of the signs of a cult. The members of the family have to have an unyielding level of devotion to the head of the family. (Akin to a cult's obsession with their leader.) One of Akito's statements to Toruh says how an outsider could never understand their ways. (Their radically different worldview puts them at odds with mainstream society.) One of the older women of the family tells the rooster that if Akito wishes for one of the zodiac to die, he needs to let Akito harm them, such as what happened to Rin. (Double-whammy: More devout members enforce the rules and the leader is able to escape punishment for such actions.)

Yuki, Kyo, Hatori, Hiro, and Haru are all shamed and guilted, to keep them in line. (Another cult sign.) Yuki is psychologically traumatized. Kyo's blamed for his mom's death, though she may've been unstable in the first place. Hatori has the whole thing with Kana on his shoulders. Hiro feels guilty for being unable to help Kisa. And Haru feels guilty over what happened to Rin.

The whole thing where the zodiac members go back to the house for the rest of their lives also counts. (They spend an extraordinarily large amount of time on group activities.) Rin, with her attempt to break the curse, is punished harshly. (Members are discouraged or punished for thinking or acting for themselves if it is against the cult.)

Tohru was bullied much more heavily than she lets on in flashbacks.
First of all, it isn't much like Tohru at all to keep grudges or dwell on tough experiences. The only reason we know about the rice ball incident at all is because it related to her problem at the time of feeling excluded from the Sohmas. When she gives her speeches of encouragement to other people it's usually that she quotes her mom and then tacks on a little bit of her own opinion. However, the first time she explained what Kisa was going through, not once did she mention "My mom used to say." Her relating to Kisa was way too specific to be just her theorizing on what she might have been going through, too. I think the game of Fruits Basket was just one part of the bullying, and Tohru and trouble telling her mom about it. This is why Kisa and Tohru became to close so fast, they could understand each other.

Hanajima's brother Megumi owns a Death Note.
He can curse people based on their names. Sounds familiar?

The Sohma Family's Involuntary Shapeshifting may be a Freudian Metaphor.
This is not so much a guess as a joke. Having only seen six episodes of the anime by the time this is written, nobody has had their sexual orientation brought up, and it is established that it takes a hug from the opposite sex to bring out the animal in them. With that in mind, one might see the Sohma Curse as a case of Something Else Also Rises. Of course, this all assumes Freud Was Right to begin with, which is likely not the case in a character-driven manga such as this.
 * Although it's a good idea, the curse is triggered by exhaustion (the body becoming weak) as well as the hug from the opposite sex.

Tohru is Haruhi Suzumiya.
How likely is it that random Bishounen would let her live in their house? Reality warping has to have a part in that.