Blade Runner/WMG

Deus Ex is set in the same universe as in Blade Runner.
The subliminal advertisements in L. A. seem to suggest that the government wanted people to emigrate to the off-world colonies. The Grey plague from Deus Ex was a conspiracy to get even more people off planet Earth. Also,  using a more advanced version of the technology used to make replicants.
 * This decends into Fridge Horror when you take into account that in Deus Ex LA and South California have collapsed into the sea. If LA was like the LA in Blade Runner, that disaster got slightly more horrific.

Replicants are the prototype of Battlestar Galactica's Cylons.
Renegade Replicants caused significant deaths. Eventually, public outcry led to the complete prohibition of androids. Cylons were invented to fill the void, and Replicants were completely forgotten... until the Cylons rediscovered that technology.
 * Or maybe they're the latest repetition in the cycle.
 * Which would make Gaff a distant ancestor of the Adama's. And that's awesome.

Deckard is Indiana Jones, age 120.
Indy's longevity is the result of drinking from the Holy Grail.
 * Or of him being a Replicant, as detailed below. Of course, this makes it clear that the Replicants are not of human origin. They're much, much older then that. They've been with us since time began. They're probably vampires.

Deckard is Han Solo.
He blasts off for a galaxy far, far, away sometime after Rachael dies.
 * Using the Time Machine from "Back to The Future", since it's also a long, long time ago.

Deckard is a replicant.
Word of God decided he wasn't a replicant at first, but after the Director's Cut has declared that he is, in fact, a replicant.
 * It varies depending on which version you watch - there were debates among the creative staff (including the actors) during production about whether Deckard was a replicant. The original theatrical cut was deliberately edited to make him human. Ridley Scott has confirmed that as long as he gets a say in it, Deckard is a replicant.
 * Deckard is a Replicant. He is defeated by the other Replicants so easily because he is a less aggressive domestic model designed to function in society rather than work as a slave. Perhaps he is a test to see if replicants can function in society if given a history first.
 * Contrary to popular belief, it wasn't confirmed in the Final Cut. As long as it's not explicitly stated that Deckard is a replicant, then it's up for debate -- especially here. The unicorn dream (the key pro-replicant evidence) can be interpreted in multiple ways (see below for one).
 * For the record, both screenwriters view Deckard as human.
 * The Unicorn is one of Gaff's -- The chap following Deckard around and apparently making sure he does a job properly. Part of the the 'unicorn at the end' thing is that he basically allows Rachel to live - Deckard's apartment has been broken into, after all. So he's making a point. He's letting her live, since no-one else knows she's a Replicant too. He could have killed her, but he didn't, and it also adds fuel to the 'Deckard is a Replicant' argument, with the 'you've done a man's work, sir' line. That's the official answer from the 'Future Noir' making-of book.
 * In the making-of documentary, we see that when Gaff says "You've done a man's job, sir," he actually goes on to say "But are you sure you are a man?"
 * Another thing to consider. Deckard's apartment is covered in photographs. Think about how obssessed Repicants seem to be with photographs as "proof" of their existence.
 * He also seems to figure this out. When headed to Sebastian's, he calls himself an "old friend." Sebastian doesn't have human friends, and Deckard knows this.

Deckard isn't a replicant...
...because replicants don't exist. They're bogeymen to keep Earth's populace in line. Every so often, a few vagrants are picked up, brainwashed, given a course of steroids, and set loose on the streets for the Blade Runners to hunt down. The Blade Runners are similarly brainwashed to keep them from questioning the events they witness. Roy Batty dies because of sustained damage and exhaustion. If replicants were real, the makers would include a kill switch, or at least an "off" switch, in case they escaped. So either they don't exist, or the geniuses at Tyrell are morons. Word of God said he wasn't a replicant at first, but following the Director's Cut, declared he is, in fact, a replicant.
 * In the beginning of the movie, didn't the police chief talk about the need for secrecy? The public at large didn't know anything about the escaped replicants; what good does it do if the only people who know about escaped replicants are the replicants and a few cops? They wouldn't even be effective as bogeymen - the only thing crazy replicants would keep people from doing is going to the colonies, and the Earth government wants people to go.
 * Oops, looks like replicants exist after all. The first draft of the screenplay used their original name, "androids".
 * That's what they want you to think! (And Blade Runners seem to be public knowledge, so rogue replicants should also be.)

There are no replicants because they are all people kidnapped and brainwashed into being slaves.
They aren't bogeymen, just a way to get slaves. They have a four-year lifespan because they were given a virus or chip implant to die after four years as a slave because the brainwashing starts to destroy their minds. That's why some go crazy and have to be hunted down by the Bladerunners.
 * So, what you're saying is that Replicants are kinda like a precursor to Warhammer 40K's servitors?

The Blade Runner universe is the precursor to the Warhammer 40000 universe.
Either Gaff or Dr. Tyrell may be the future Emperor. (Tyrell faked his death with a replicant lookalike.) The Tannhauser Gate is an early attempt to access the Warp, and the replicants' revolt was due to some being possessed by Daemons.
 * Or they are the Iron Men that shattered the first human galactic society

On the other side of the world, the Japanese have Replicants of their own
Except Japan is doomed. And everyone's strangely happy about it.
 * Unless their Neo Kobe Pizza doesn't come back up.

Deckard is not a replicant, but a unicorn.
And he finally knows it when he sees the origami unicorn. Many unicorns shapeshift to look like humans...
 * Gaff is the wizard who turned him into a human.
 * And then Deckard gave into peer pressure and went to Candy Mountain, waking up in a Crapsack World.
 * This is why he's attracted to the weirdly virginal Rachael, and also why he behaves like such a brute. Though pretty, classical unicorns weren't gentle creatures.
 * Wait, Deckard is an otherkin?
 * No, Deckard had sex and can function at least somewhat normally in human society.

Deckard eventually realized that some of the things he was sent to retire weren't replicants...
They were people who couldn't make their payments to Geneco.

Deckard is a precursor to later Repo Men, who are better protected and less "human".
After the whole Replicant thing blew up, people settled for bio-engineered organs, dealing with immediate needs first. The people left on earth, who didn't skip town to head offworld, were perfect victims for the organ failure epidemic caused by generally crappy conditions on the surface. So, rather than put all their money into Replicants, the guys at Tyrell decided to just sell the transplant organs. But first, they needed to rebrand. Voila! Gene Co.

Everyone is a replicant
There is not a single character in the movie for whom there is convincing evidence that they are not a replicant. No one has passed a VK test. Most so-called humans behave in a rather inhuman manner. Few display any real emotion, and even replicants can easily fake what they do show. (Twenty or thirty questions with a laser pointing at your eye? No way a viewer will notice the difference).
 * This is part of the point of the film. The idea is that the city and its high-tech, gritty urbanity are so dehumanizing that humanity is less human than the replicants, who come from another, presumably more rural space.

Persocoms in America were combined with the artificial organs industry to create the Replicants.
This is why they were similar to humans on both the outside and the inside (so that the now-mature organs could be harvested once the Replicant was decommissioned). The  only affected some of the American bio-persocoms -- turns out that allowing an immune system is useful for things other than the self-destruct sequence.

Blade Runner takes place in the future of the Watchmen universe.
Genetic engineering? A ridiculous amount of Aztec and Egyptian architectural symbolism? Hm.
 * And the Tannhauser Gate is the product of Veidt's teleportation technology. The airships seen everywhere are derived from Archie. The Spinners are a miniaturization of the same flight technology.

Blade Runner takes place in the future of Metropolis...
...making certain similarities to said movie more than just Shout Outs. Rotwang's robot was of course the very first replicant.
 * Well, Metropolis takes place in 2027, and Blade Runner is before that, in 2019. It could be explained, though, by saying the dates mentioned aren't the real ones.
 * Or, since Metropolis is a German film, Germany lacks any replicants, and is only now looking into the technology. Actually, it could be argued that the robot in Metropolis is a better replicant than any in Blade Runner, because it can change how it looks to different people.

The unicorn represents Rachael.
Unicorns are symbols of femininity; they are strong, they are beautiful, they are unreal. Much like Rachael was when Deckard first met her: very feminine, very no-nonsense, very beautiful, and a replicant. The unicorn in Deckard's dream (or daydream) is an abstract representation of how he views Rachael. Regarding the unicorn origami at the end: Gaff's origami had only ever been used to represent people throughout the film: in Bryant's office when Deckard refuses to come back he makes an origami chicken, symbolising Deckard's cowardice; later in Leon's apartment he makes an origami human figure with a big dick, representing Deckard in his element as a detective. Gaff made the origami unicorn at the end to represent Rachael: he saw her as Deckard saw her, and so he decided to let her live and allow them to flee north without coming after them. When Deckard picks up the unicorn, remembers Gaff's parting line "It's too bad she won't live, but then again who does?" and smiles, he is acknowledging that Gaff is saying to him, "Good luck for the time you'll have together."

Some Wild Mass Guessing from Roger Ebert.
"Even one character we can safely assume is human, the reptilian Tyrell, czar of the corporation which manufactures replicants, strikes me as a possible replicant. And of the hero, Deckard (Harrison Ford), all we can say for sure is that director Ridley Scott has left clues in various versions of his film that can be used to prove that Deckard is a human -- or a replicant...What I have always wondered is why the Tyrell Corporation made their androids so lifelike. Why not give them four arms and settle the matter, and get more work out of them? Is there a buried possibility that Tyrell's long-range plan is to replace humans altogether? Is the whole blade-running caper simply a cover for his scheme?...The "human story," as I think of it, involves practical tests to determine if an individual is a replicant or not, and impractical tests (such as love) to determine how much that matters to (a) people, if they are in love with a replicant, and (b) replicants, if they know they are replicants. This has always been a contrived problem, easily avoidable in practical ways, unless (as I suspect) the Tyrell Corporation has more up its sleeves than arms." -from his "The Great Movies" review of "Blade Runner: The Final Cut"

Blade Runner, Alien & Soldier(The Kurt Russel film) are all set in the same universe around similar times.
This theory was hinted at by Ridley Scott himself in the Final Cut commentary, as he directed both Alien and Blade Runner. Also the script for Soldier was written by the Co-Writer of Blade Runner, and does consider it a spiritual successor to the film.

Blade Runner, Alien, Prometheus and Terminator are all one big thing.
Okay, stay with me... "Hyperdyne Systems", the creators of Bishop, are, in fact, Cyberdyne systems, rebranded. The Terminator temporal war was eventually settled when a very smart soldier from the future convinced Sarah Connor to sue Cyberdyne out of existence by presenting pieces of the Terminators sent to kill her as evidence in open court. Yeah, its crazy, but the hysteria over it set Cyberdyne back 30 years, so Skynet never really gets built. But this clues Weyland into the possibility of FTL travel (if time travel is possible, so's FTL), which sets us on course to Prometheus. The Tyrell Corporation gets bought out by the 20-teens by Weyland after a scandal involving the death of its founder, hinted as being at the hands of a rogue Blade Runner. Weyland gets into the android business, and Hyperdyne is their chief competitor. A nuclear war happens somewhere in there, anyway, but not nearly as bad, because even in the absence of killer computer systems, humanity is still a bag of jerks.

No one is a replicant. Deckard is just a deranged serial killer.
All of the talk of replicants is just Deckard's perverse fantasy justification for doing what he likes: killing defenseless sex workers. Gaff is the voice in his head that tells him to kill, and Rachael is just some poor secretary that will be his next victim...which is why she won't live.
 * And his encounter at Tyrell Co. headquarters with the head of the company was just a therapist playing along. When the asian man serving him at the restaurant refers to him as "Blade Runner", it isn't a term for people who kill replicants...it's a serial killer nickname for him. All scenes without him in them are his dreams.

Blade Runner's Earth is Earth-That-Was.
2019 L.A.'s melting pot of Asian and Western cultures hints at the early beginnings of the Anglo-Sino Alliance. Also, the migration of humanity from Earth to the Off-World colonies suggests that Earth will soon become abandoned as the Alliance establishes its government Off-World, forming what will later be known as the Core Worlds.
 * which explains a scene from Serenity where there is a flying 1970s-vintage Ford. The core worlds would certainly keep all the replicants for themselves or else not bother to tell any of the outer worlds that the military is made up of x-percentage of replicants.

Deckard has been incepted by Gaff to think he is a replicant.
The unicorn is a false memory, planted deep into his subconscious. Once Deckard completes his mission, Gaff pulls his trump card (the origami unicorn) on Deckard to show him that he has murdered his own kind in cold blood. If he offs himself ( "But then again, who does?"), that will get him out of the way and Gaff can become the top blade runner.

So why make Ridiculously-Human Robots if it's going to create so much confusion?

 * Maybe replicants normally work alongside humans, who wouldn't like spending their time surrounded by obvious robots or would refuse to cooperate with them.
 * This is the most probable explanation for the "Pleasure models"
 * Maybe it's to allow replicants to change careers; an obviously robotic miner is one thing, but a robotic sex worker or waiter would be less effective.
 * Maybe it's a conspiracy to replace humans.
 * It's probably not such a big problem on the Offworld Colonies, where the Replicants are implied to all look very similar.

None of the movie actually happened beyond the interview at the beginning
The whole thing is a shock-induced hallucination being had by Holden after he was shot during the interview at the beginning of the movie.

Sebastian is a Replicant, and his condition is the result of extending his lifespan.
If he were able to help Batty and Priss, they would age in the same manner as he does.

Gaff is a pimp.
He wears fancy, expensive clothing that is clearly non-regularion, carries a cool looking cane, and walks with a noticable limp like an old-school pimp.

Gaff is a Replicant.
This goes with the theory that Deckard is also a Replicant. The origami unicorn was left behind not only as a clue that Gaff is letting Rachael go but also that he knows that Deckard is a Replicant (due to the unicorn dream Deckard had as seen in the Director's and Final Cuts). Why would he let them go if he knew they were Replicants? Because he's a Replicant himself; he sympathizes with their troubles because he has them as well. He also has a vibe throughout the film that he doesn't like his job any more than Deckard liked being called back (he also didn't like the idea of bringing Deckard to be forced into the job because he's an accomplice in what is essentially the murder of other Replicants). The reason he was able to go on for so long without suspicion was the fact that he walks with a cane. As far as other characters know, no Replicant would have that kind of physical limitation even if one was made (with an experimental Replicant like Rachael, and also Deckard if you buy into that theory, who's to say Tyrell hasn't made a Replicant with an obvious impairment like having to walk with a cane?). There's also his last line in the Director's and Final Cuts of the film: "It's too bad she won't live. But then again, who does?" The way Gaff says it seems to imply that he's accepted his fate that, as a Replicant, he doesn't have much time to live and wants to help them out before he dies.