Baccano/WMG

Jacuzzi's Chaste Hero status is at least partially a defense from Nice's fetish.
His logic is that kissing leads to making out, making out leads to sex, and sex leads to "Dear, would you mind putting this bomb around your ****?" I'm sorry, is this too soon?
 * Not at all, in fact it sounds good enough to be canon.

The main character isn't from Baccano. Instead
...It's

Rachel is related to Huey Laforet, possibly his Homunculus
She has the same gold colored eyes as Huey and Chane, and her hair, color aside, is VERY similar. She also lacks a last name.

The Japanese tourist Firo talked to in the Frame Story of The Rolling Bootlegs is Ryogho Narita with some personal details changed in the publication
Upon returning to Japan, he figured he could write a book on it to make some quick cash. Sometime later, the first volume of Baccano! is published. Firo finds out and is not only amused by it all, but offers to give him more of the story for a cut of the profits. The rest is history.

Claire Stanfield is faking his solipsism
He seems to care about certain other people like the Gandors and Chane, which isn't consistent with a genuine belief that everyone else is a figment of his imagination. His whole speech about being the only thing in the universe was just a calculated effort to unnerve Ladd.
 * If there's only one thing fandoms prove, it's that it's possible to care about fictional characters while stilling knowing that they are fictional. Instead of the Corleones and Sydney Bristow, Claire just prefers Those Lovable Mob Brothers and Sexy Mute Knife Nut.

Ladd's promise to kill Lua is rooted in insecurity from his past.
It's possible that due to the murder of his former fiancee, Ladd is afraid that if he doesn't kill Lua himself, someone else will and he doesn't want to go through that again. His constant reminder to her that he's going to be the one to kill her is as much a reassurance to himself as it is to her.

Isaac and Miria are actually fae.
The two of them clearly have no real understanding of human behavior, as evidenced in the haberdashery scene. They hold onto a weird orange and blue mortality in which crippling a family's finances is a good deed that will make up for the evil deed of stealing chocolate bars. They also fail to understand thresholds, thinking that no one will be able to cross into the museum without a door.

Chane isn't Huey's daughter, she's his homunculus.
Pretty simple: she's ridiculously adept at fighting, something Ennis also was, she doesn't seem to have a mother, and most importantly, he's able to affect her to the point of taking away her voice. At no other point in the series do we see immortals being able to affect anyone like that except with Szilard and Ennis. I'm American, so I haven't read the books at all, so I don't know if she ever ages. I assume she does, since otherwise this point would be more obvious, but it could be simply that Huey wasn't as adept at creating his homunculus as Szilard was.
 * Chane does age along with Claire, so she isn't an immortal (or at least a perfect one); her abnormal combat abilities is not a valid reason to consider her inhuman since many killers and assassins in the series have Charles Atlas Superpower in their veins: Ladd, Graham and Claire are all ridiculously powerful while being just human, especially the latter as he is the strongest character in the whole series. Most of all, spoilers are scattered through this article to confirm why Chane was born, and who might be her mother, and even Chane herself has kids of her own as the time goes. I say it's completely Jossed the idea of her not being human in any way.

Huey is the owner of or somehow affiliated with Nebula.
Possibly, his first mentor, Rene Paharmadeus Brinvilliers, in the 1934 events is a head researcher for the recently (at the time) built Lab: Nebula Sky Laboratories, financed by Senator Beriam himself and with the main subject of research being the Homunculus and Immortals. Taking into consideration that Huey is, maybe this might trigger Nebula Sky's growth and eventual shortening of its name to Nebula.

Isaac and Miria are at least semi-responisible for everything that happens in the series from the 1930s onwards.
You know how in the opening credits there's a Joker card with them on it? Well, that refers to more then their status as the comic relief. The term 'joker in the pack' refers to "someone or something that could change a situation in a way that you do not expect", which is exactly what they keep on doing, whether it's accidentially making the mobsters immortal, kick starting Ennis' character development, or just indirectly effecting events by being in the right place at the right time, eg them getting run over delayed Ennis and Szilard's arrival at the meeting, which had a knock on effect which led to Ennis meeting Firo and failing to find the man she was looking for before Dallas and co, nicked off with the bottles, and we all know where that led. Basically a butterfly flaps its wings and causes a hurricane, Isaac and Miria do something random and awesomeness happens.

Although the main character of the events surrounding November of 1931 changes depending on which story is being told, the main character of the Baccano! anime is Carol...
...Because the anime is a story about stories, and how a young girl learns something interesting about the nature of meta-discussion.

The series takes place in an alternate universe, perhaps one where France won the Seven-Years War
There are an awful lot of weird names in the series, but in particular, a lot of them are vaguely-French sounding (i.e. Isaac Dian) and there's also the odd thing that Huey is imprisoned in Canada seemingly under orders of American government officials. So, my conclusion is that in this universe, French Canada took over the United States. And for some reason, Alchemy works.
 * Newfoundland didn't join confederation (IE become part of Canada) until the early fifties, until then it was a separate Dominion under the British Empire. Besides, he could be under a kind of joint arrest between the Empire and the US, or a sort of Guantanimo Bay (technically in Cuba) setup.

The world really DOES revolve around Claire Stanfield
After all, we already know that Baccano is just a fictional story. Maybe there's some kind of self-referential thing going on.
 * The world will end in 2012 because that's the year Claire finally kicks the bucket.

Specifically, the world is a video game, and Claire is the player character.
While he knows none of the other characters are really real, he’s still taking a basically “good karma” route. Thus, he responds well to friendly characters and those invoking Video Game Caring Potential (particularly the “family” and “love interest” NPCs), yet easily responds with the traditional player response to unsympathetic /antagonist characters.

This is also why he’s an ordinary human who can still effortlessly take on hordes of mooks and ostensibly more powerful bosses. On top of the usual player character advantages, he’s either good at the game, or simply reloading when he screws up (or both). This is why, despite his ostensible mortality, he’s never worried that he might die- if Claire dies, he can simply reload from last save and do it right.

The anime exists in the future of the world of the books.
Sometime after the events of Durarara, a documentary series is made about the Daily Days' most significant reports, and Isaac and Miria as themselves are invited to do an actors' commentary. The director loved their goofy little Foreshadowing comments over the Next Episode previews, and left them in. To sum up the series' connection to the fictional "reality", it's a release of a series about a report on the events that took place in the books, which in turn don't exist within their own continuity.

Baccano takes place in the same universe as Eternal Darkness.
Because.

Baccano takes place in the same universe as Fullmetal Alchemist
No one's said this one already? I thought it was obvious, with the alchemy and all...
 * Not to mention the presence of homunculi.
 * It could also explain Kimblee's own outfit: he was obviously working with Ladd besides being in the army... at least before getting thrown in jail.

Baccano takes place in the same universe as Chrono Crusade.
...It's not that much of a stretch.
 * When you take into account the not neccessarily evil demons (sorry, "Devils", as is the common parlance of both series) and the perversions of science and scripture (Devil Summoning/Alchemy)

Claire Stanfield will die on July 7th, 2007
Actually, "die" isn't the proper term here; rather, he'll decide that he's tired of being Claire Stanfield and decide that he wants to be Haruhi.

Claire Stanfield IS Haruhi, who has become aware of his/her powers
Haruhi eventually learned the truth about her self and decided to make the world more interesting. Her first action was to create aliens( the demon) espers (people like Ladd and Graham) and time travelers (the immortals, via The Slow Path). She also decided to genderbend herself, because why the heck not? Its mentioned that Claire is adopted, and since he's so willing to throw away the name 'Claire' its probably not the first time he changed his name. And all those stunds he does? He's more badass than anyone else in the series. In short, he wasn't kidding when he said 'the world is mine'.

Claire Stanfield is an ancestor of Marlo Stanfield.
Okay, Marlo's a Complete Monster, Claire isn't, but they have a name and a lot else in common. Marlo is separated from previous kingpins due to his lack of remorse or mercy. What's more, he is completely obsessed with himself and his own reputation, he has utterly no conception of any other life but what he's doing now, and he shows impressive fighting skills against multiple armed opponents. Marlo is Claire distilled through generations of Baltimore drug culture. Maybe Claire had a fling with a black girl in Baltimore in the '20s, Lamarck Was Right, and the rest is history.

The Heiwajimas of Durarara are descended from Huey Laforet, possibly through Chane and Claire.
Huey's descendents have an established tendency to look just like him. Except in eye color, Kasuka Heiwajima looks... a lot like Huey, and almost exactly like Chane and Claire's great-grandson Charon Walken, a fact pointed out in-universe. This ancestry also helps to explain Shizuo's Super Strength.

Baccano! takes place in the same universe as Highlander.
The alchemist was trying to distilling immortal blood to make the immortality potion.

Baccano! takes place in the same universe as Portal.
And Chell is an Immortal. Getting shot at by turrets, she loses her weight in blood throughout both games only to be healthy and unhurt a moment later. The only logical explanation for her insane Healing Factor is that she's an Immortal!

Rachel and Graham Specter are siblings.
I got to thinking this when I saw this image. Isn't there a similarity, both in their hair colours and hair style? Besides, we don't know Rachel's last name either, or anything about Graham's father...

I-zack and Miri-a make a porno.
Because that sounds like something they would do.