Whose is the unicorn dream?
Deckard is an original creation. None of his memories are his own, they were given to him. Someone, some human, must have had the unicorn dream before he did, and then it was implanted in replicant Deckard.
If Deckard's a replicant, who knows Deckard's a replicant? Who's part of this conspiracy?
Let's start with Bryant. It looks like Bryant is on some kind of established terms with Deckard. "I need you, Deck. I need the old blade runner," and dang it feels strange to write that noun phrase out in lower case. Bryant and Deckard are not on friendly terms at all, adversarial, but they clearly have some years of shared history. Could it be that all of this just a charade by Bryant? Is he acting the part of a long-term "friend"? Is he being subtle? Is it in character for this replicant-hating police captain to work cheerfully with, and provide sensitive information to, a known replicant whom he only met a few minutes ago?
In which case, who's Deckard based on? Gaff? Except Gaff doesn't seem like a blade runner himself; as a blade runner, you'd think Deckard would know the other blade runners in his area, but he doesn't seem to recognise him when he comes to arrest him at the start of the film. (Or does he?) Also, Gaff walks with a cane, he isn't physically fit to hunt replicants. But perhaps he's retired. Or is Deckard based on Holden, then? This would make a lot of sense, given that Holden was the first choice for the job. Or maybe a combination of them, and perhaps other blade runners not seen?
Alternatively, is Bryant in the dark? He doesn't know Deckard's a replicant at all. In which case, when, exactly, was Deckard built? We know that legal replicants can't have a lifespan of more than four years. Nor can a legal replicant's lifespan be extended; Tyrell seems quite earnest about this. But Deckard's operating on Earth, which is highly illegal, so who's to say he hasn't been operating for years longer, or even decades? ...Except that, as a replicant, Deckard is clearly at the cutting edge of the art, equivalent to a Nexus 6 or even a little way beyond. Let's say Deckard can't be more than a few years old. So does that mean Deckard worked with Bryant for a few years, without either of them ever realising Deckard was a replicant, then quit/retired, went to a noodle bar, read a newspaper, then got dragged out of retirement?
Or... was there a human Deckard once, whom the replicant Deckard replaced relatively recently? Was this the man Bryant worked with? And then he, and his memories, including the unicorn dream, were used as the template for the replicant Deckard whom we see in the film, who replaced the human Deckard, and Bryant was never told, and never noticed?
Given this admittedly large leap... When did the switch take place? Months or years back, while human Deckard was still in gainful employment? Or relatively recently, during his brief retirement? (As long as I'm throwing ideas at the wall, who says "Deckard" has to be entirely human or entirely replicant? Could a switch have taken place, shock, horror, some time during the events of the film? Perhaps as late as the scene where Deckard is on the roof watching Roy die?)
If Deckard's a replicant, but there's a human Deckard at the back of all this, where is he? We can assume he's retired, and that could mean either literally or figuratively or both. Is he dead? If so, who knows about that? How could Bryant, for example, not know about that? How did he die? Did he die on this exact case? Perhaps by the hand of one of the escaped Nexus 6 models? This seems doubtful, given that none of Priss, Roy, Zhora or Leon (or Rachel) seem to recognise him... And the timeline seems narrow. Or if Deckard's dead for some other reason, then how did he die? When? Can memories be recovered from a corpse?
Or, now edging a fair way beyond speculation into fan fiction, if the human Deckard's alive, where is he in reality? And for which of the numerous possible reasons has he not taken this case up himself? Is he offworld? Is he still in Los Angeles, but unable to cooperate? He retired for a reason, after all. What was that reason, anyway? Blade running is extremely dangerous, so maybe he's injured, like Holden. Or maybe he's just unwilling? (When replicant Deckard is asked whether he's ever retired a human by mistake, he says "No." Maybe it's true that replicant Deckard never has... but if human Deckard had, why should replicant Deckard be granted that particular memory?) And if human Deckard is unwilling, but his memories were the ones used to create the replicant Deckard, did the human cooperate in this, or resist it? Maybe they just said, "All you need to do is sit in this machine for two hours, and then we'll be on our way, and you never need to hunt replicants again."
"I was quit when I came in here, I'm twice as quit now." Get it? There's two of him, and they're both retired.
Questions. If Deckard's a replicant, who commissioned his creation? Does anybody in the LAPD know they're employing a replicant? We know (or do we?) that Gaff knows that Deckard's a replicant, because of the paper unicorn. It seems a pretty sure thing that this conspiracy must involve Gaff, maybe even start there. But: remember that using a replicant on Earth is an extremely serious crime, subject to the death penalty. Although the opening text is unclear, this penalty may apply to both the replicant and the person who let it loose. Releasing a replicant on Earth, even if it never discovers or suspects its own nature, is horrifically dangerous to you, the one who released it. And that's before the replicant you created gets given a gun and set to the task of identifying and exterminating other replicants, and that's before the replicant goes off-reservation and starts thinking for itself. So Gaff's part of this? Is the entire replicant-hunting division of the police operating outside the law, and this has been the case for years, or is this a relatively small operation, testing out a new idea? Whose idea is it? Why did this seem like a good idea? Do we think Gaff's that unbalanced?
Why does Gaff pass hints to Deckard? What's his agenda?
Who built Deckard? Okay, this one's relatively easy. If Deckard's a replicant, he's the most convincing replicant in the film. Only the Tyrell Corporation builds Nexus 6 models, although doubtless the corporation has competitors offscreen somewhere.
Asking more out of curiosity than anything else: How long does it take to construct a new replicant? Is it feasible that Deckard could have been built (or is it grown?) in only a couple of days after Holden was shot?
For that matter, who paid for Deckard to be built, and how much did he cost? If they're anything like artificial owls, Nexus 6 model humans must be outstandingly expensive. Who's got that much cash? Gaff? Others in the LAPD with whom Gaff conspires? Retired human Deckard? Maybe the unicorn? Or, even more interestingly, could Tyrell Corporation have taken the job pro bono? Now, what could motivate that?
Now we're cooking. The Corporation must build many replicants for numerous purposes. So, does Eldon Tyrell himself know about Deckard in particular? Suppose he does. Suppose Tyrell takes an interest, and/or this was a particularly special job demanding his personal involvement. Then what's his agenda? Did he meet Gaff? How did that meeting go? Was all of this, in part, Tyrell's idea?
What's really going on when Deckard puts the Voight-Kampff test to Rachel while Tyrell observes? If a Voight-Kampff test can detect even the most advanced replicants, then is it feasible for a replicant to be able to reliably administer this test to other possible replicants? When Tyrell says "I'm impressed," what is he impressed by, really? At first glance it looks like he's impressed that Deckard (representing blade runners in general) managed to get the correct result after testing Rachel, whom he considers to be a difficult subject for the test. At second glance it looks like he's impressed that Rachel was able to flummox the test procedure for such a long time. If Deckard's a replicant, though, and Tyrell knows it, it swings back the other way.
The question of how Tyrell feels about the hunting and "retirement" of rogue replicants was already open, and remains very interesting. But if Deckard's a replicant and Tyrell knows it, then how does Tyrell feel about setting replicants to catch other replicants? Does this further or obstruct his grander goals? Is he just overjoyed at the perfect larger-than-life cyberpunk post-noir tale that his replicant creations are spontaneously acting out in from of him? I can see that.
What if Tyrell is a replicant? We get to go back and ask all of these questions again, that's what! We also have an opportunity to get paranoid: is Gaff a replicant? Is Bryant, is Holden, is Sebastian? Is Zhora a human?
And at the end of the day, the whole idea sure went sour. The four rogue Nexus 6 models are dead, but now Deckard and Rachel have escaped. Does another hapless replicant get sent after them in turn? And so on, until it turns out that there's not a single real human left on Earth...
If Deckard's a replicant, then it seems as if a significant chunk of the film is just outright missing. Honestly, this fact in itself could be construed as a very strong argument for him to be human, because if a thing were such a significant part of the film, surely it would be in the film. On the other hand, it also seems that deliberate ambiguity, and the hugely important discussion about which way it could go, is the "answer" to Blade Runner. So all I'd add is, if you love ambiguity, and open questions, then for Deckard to be a replicant opens up plenty of ambiguity and plenty of open questions.
They're making a sequel. Harrison Ford is on board. He's also visibly decades older. Given the theories above, this alone does not necessarily mean that Deckard of Blade Runner is a human.
There are a lot of different directions that this could go. Blade Runner's ambiguity is one of its great strengths, and I don't see how the sequel could possibly be made without somehow disambiguating the original, which would be regrettable, but I'm willing to be surprised.