A movie about a replicant that doesn't know it's a replicant hunting other replicants would be fascinating.
A movie about a sociological experiment, concocted by a scientist and a police department, to see how a replicant that doesn't know it's a replicant reacts to hunting other replicants would also be fascinating.
But the cold, hard fact is that that's not Blade Runner's story. It's subtext, it's flavoring for the themes running through the movie, but it's not the mechanics of the plot.
It's seeds sprinkled into the background of the plot, like how Dana's constant eating and hickey on her neck in Poltergeist hint that she's pregnant, something that plays into the theme of the movie of secrets hidden under the surface of the suburbs. The difference is, of course, that Dana is just a side character and sits out most of the plot.
Deckard being a replicant fundamentally changes the point of the story, yet is something the movie doesn't actually engage with except on one occasion, when Rachel asks him if he's taken the Voight-Kampff test. Compare Blade Runner to Ex Machina or the Westworld television series. All three stories are about the fine line between humans and "robots," with Ex Machina presenting a human being that doubts their own humanity for a moment and Westworld presenting a human being that discovers they're actually a host. It's actually part of the plot in two of these three to explore how a human being would react to doubting their humanity and the fallout from that.
For Deckard to be a replicant means that we never get Deckard's feelings on the matter or the feelings of other replicants toward him or even the feelings of the humans that set Deckard in motion (Tyrell seems indifferent to Deckard, although he's passionate about Rachel not knowing he's a replicant, and Bryant's only feelings on anything seem to be arrogance and bloodlust). It's dramatically inert, with none of the characters reacting to what in any other story would be the cornerstone of the plot.
To see a story in which a Blade Runner turns out to be a replicant, struggles with it and struggles with how other replicants react to him, play the 1997 video game (in a few of its multiple endings, in some the main character is human).
Multiple outcomes, fantastic design and even if its tech has aged quite a bit, still one of the best adventure games ever made.