UTAU/WMG

Every single fan vocaloid/UTAU that exists will be canonized.
Because Yamaha, whoever made UTAU, and some anime studio will partner and make a High School Spinoff Anime, called Song Academy, where the official Yamaha ones will be the main characters (including the dreaded non-Japanese ones, with Sweet Ann as the English teacher), their opensource counterparts will be secondary ones (with the exception of Kasane Teto, that became kinda popular, even more than the friggin' Defoko), most popular fancharacters, like Akita Neru and Yowane Haku will make their (usually comedic) appearances; and all the not so popular ones will be background, or single shot characters. Unless, of course, they are held high in popularity polls.
 * I would so love that...

The past divas in Synchronicity are all Vocaloids/UTAU.
At least it's what the community believes. In this video, you can see the flashback with all the past divas, and they seem to be:
 * Kasane Teto
 * Macne Nana (the one with a rose on her hair)
 * Prima (the black-haired woman with a bun)
 * Lola (the blonde in black)
 * Sweet Ann (the blonde in white)
 * Miriam (silver-haired girl)
 * Sonika (it's the blurry girl in the furthest point)
 * Nagone Mako (another guess,﻿ it's the one with loose, straight black hair)

And there's one more, with short hair, which could be another UTAU, but not sure of who she could be.
 * This Troper guesses it's Supune Niku, due to her brown hair and yellow clothes, but this UTAUloid isn't even confirmed to be female (her creator said that she's perhaps a female), let alone a diva.

UTAU is actually a plan by an evil mastermind to gather all voices in the planet.
I know it's a little too wild for WMG, but you can see the connection. Ameya actually doesn't really have idea on what's going on; even though he created UTAU, it was a plan all along.

Tree is a scientifically altered being, not a magical one.
That doesn't really explain how he/she/it started to sing randomly, but it'd be really ironic. Pherormonal experiments, anyone?

The more popular UTAU fandom gets, the more random it gets.
It might have started with the VIPpaloids, which characteristics were given by 2ch VIPper posts, but some of them just sprouted of nowhere. A voicebank with only gibberish/garbled phonemes? Check. A dog head in a snake's body? Check. Singing plant/hybrid girl? Check. A cabbage with legs? CHECK. Furry UTAUloids? PLEASE MAKE IT STOP! The randomness burrrrrrns.
 * Don't forgot about the BACON.

Sukone Tei isn't really a VIP@2ch troll
She's just product of boredom and a singing masturbation joke.

UTAUs are made from the souls of humans who don't survive the Vocaloid conversion process.
The Vocaloid conversion can be--and usually is--fatal. So, everyone who dies in the process has their soul transferred into a robot shell known as an UTAU. Each UTAU is then mass-produced and distributed freely as a sort of pre-Vocaloid starter kit. Since they're made from human souls tranferred into robot shells rather than brainwashed humans, they tend to have more consistent personalities, but their voices don't sound as clear and human-like.
 * So, how do you explain when an UTAU gets high quality renzoku and sounds like a Vocaloid?
 * Better quality shells, or frequent maintenance by the users.
 * What happens to the soul of a person who makes more than one UTAU?

Chikune Kenta's main food source is lemons.
He only becomes an evil vampire when really irritated... But he needs to feed on something. It might explain his addiction for lemons. How they substitute blood in any way, it's another story...

Sukone Tei is a transgendered Sasuke.
It'd explain her sharingan. She's from an alternate universe where Sasuke loved Naruto, but had to fight against him due to his deeds against Konoha. After having to kill Naruto (and letting his hair grow like crazy and become white in the process due to Rule of Cool and shonen hair-growing trends), he moved far away from the hidden villages to Tokyo, where he knew Kagamine Len. Thinking that Len looked like Naruto (remember, Sharingan overuse damage the user's eyes), he fell in love and declared himself to Len, only to be rejected. He did some stalking and discovered that (this version of) Len only liked girls and had a fondness for Miku. Then, Sasuke decided to transgender himself, dress in a similar fashion and become a singer. After being rejected once again by Len, Sasuke, now Sukone Tei, decided that maybe eliminating everything in his way when necessary wouldn't be a bad idea, despite the fact that he ended up disliking killing (because he had to kill his friends and lover in the ninja wars). Don't ask me about the cucumber and masturbation fondness, I don't know if I would have an appropriate answer for that.

Tei is from the same faction as Asakura Ryouko
. She's a data entity, Yandere, prefers violent/bloody methods...perhaps that sharingan is not a sharingan but something...scarier.

Ritsu must be a special VIPPERloid.
The fact he's the only VIPPERloid without at least one of his eyes being red, added with the fact he has built-in heavy ballistics (his boob missiles), might mean he was built not as a joke on North Korea's allegation of nuclear bombs, but as a way to protect Vocaloids and other UTAUloids from a possible VIPPER attack.

Deon, Mugen and Haruto Natsuko (as well as other time travelers from the future) have more onto themselves than they appear to do.
There's no way time travelers from the future come to the present simply for fun. They must have their missions on this time, but probably forgot about those, as most UTAUloids seem to generally live pretty light-hearted lives.

There are NO normal UTAUloids.
No matter how much one tries to say that their UTAUloid is normal, it isn't. They'll always have a quirkness of some sort, even if they're just Miku recolors (although, being a Miku recolor should be quirky enough on its own).

Donka Fjord is an Eldritch Abomination in the body of a cute android.
Because WMG needed that theory. Two hints for it: his "gibberish" voice and the fact that he, officially speaking, has no eyes. At all.

Alternately, ALL "jokeloids" are Eldritch Abominations in UTAU world.
Or at least Fjord, Tree and Eert.

TATARI was expelled from Hell.
His whole "fabularity" deal is just self-delusion. He was probably a bit too mischevious for Hell, and was temporarily banned as a punishment. He probably will be able to be back when he gathers wnough souls. If he gathers them, that is.

Teto's is the drill that will peirce the heavens.
You want it to be true.

Teto actually is Amy.
First of all, just look at their hair. Second... if we assume Amy is 14 as of SCIV, and we know that SCV takes place 17 years after SCIV, how old does that make Teto/Amy? 31 - Teto's official age. (She didn't physically age between the games for some reason.) Also, her chimera form may be a side effect of malfestation...