When the Directors Cut first came out it opened up the question of Deckard being a replicant. It went back and forth for a while before Ridley Scott came clean in an interview with journalist Mark Kermode and definitively said so. Case closed. Except no it isn't. The thing about sci-fi is that you have to create a world and these worlds have rules. Once you set these rules up and establish how your vision of the future works to the audience you really need to stick to them in order to get a really good film. Blade Runner has some rules and by following them through and looking at the players in the film leads to a revelation.
The replicants in Blade Runner know they're replicants. Except Rachael. She's a new type of replicant and she's been given somebody else's memories to cushion her emotional responses so she can be more easily controlled. So if Deckard is also a replicant, he doesn't know about it either. So he's the same as Rachael. It follows then that he must have somebody else's memories too. And there's really only one person who's memories he could have.
Deckard is Gaff. If you think about it, it makes complete sense. Who is Gaff anyway? I propose that he was a Blade Runner before he got injured and acquired his limp, possibly even going after the same replicants in the film. He's right up there in the cop ranks and appears to be close to Police Chief Bryant, the same relationship Deckard re-assumes when he's hauled in at the start of the film.
But notice that Deckard isn't even allowed to fly his own police car. Pretty much every time he needs to go somewhere, Gaff has to chauffeur him around. He's a real dick about it too, he doesn't talk to him or even acknowledge his presence. He just drives him about with that resentful scowl on his face. How come Deckard isn't flying himself around?
Right through the film, Gaff shows complete contempt for Deckard. Right at the start when he approaches him eating those delicious looking noodles, his way of saying "Hello" is by hitting him on the arm with his cane. All the way through he's basically a massive prick to him. The relationship makes sense - this whole endeavour of using a robot to hunt robots is experimental. If Deckard has Gaff's memories and skills than who better to keep a close eye on him to assess how things are going? And how much would Gaff hate doing this? The more Deckard succeeds, the more reason Gaff has to hate him. He's everything Gaff once was and serves as a painful reminder.
On top of all of this we have the origami. All throughout the film Gaff appears to know what Deckard is thinking. When he's getting the original brief from Bryant about how hard the job is going to be, Gaff makes an origami chicken and puts it on the desk because he knows Deckard is scared. He knows this because he would be intimidated by the job himself. Later, when discussing a visit with Rachael, Gaff makes the little matchstick man with a boner. He does this because he's already visited Tyrell and met Rachael and knows that Deckard's going to fancy her.
Then we have the final acknowledgement that can't really be interpreted any other way. The origami unicorn at the end of the film. Gaff knows about Deckard's recurring dream of the unicorn. How else could this be the case unless he also has the same dream?
Gaff says something very telling to Deckard near the end of the film. He lands on the roof right after the climactic fight which means presumably he was overhead observing the whole time, letting them get on with it to see how things played out instead of stepping in and helping. As he lands, he walks up to Deckard smiling and says "You've done a man's job". Coming from Gaff, this is the ultimate compliment. He has accepted him as his equal.
So that's my Blade Runner theory, that Deckard is actually Gaff. Might go and watch it again now.