I think the reason sequels don't work is usually what made the first so special cannot be duplicated, or the creative team involved don't know how to duplicate it. With Blade Runner I'm guessing it can't be duplicated. Everyone who is a fan should watch the making of Blade Runner. They put so much crazy, crazy amount of detail into making it. The set where the street scenes were shot was so detailed that crews from other movies on other lots in the studio would come over and hang out on breaks because it was so fascinating and felt so realistic. Even though they built models for all the buildings they put in painstaking effort and hours to keep them from looking like model buildings.
So many other things happen, as with many works of genius, by accident or as a result of unforseen conditions. Harrison Ford's working conditions, aka not getting along with Ridley caused him to give the perfectly detached performance that keeps you guessing about whether he is a replicant or not. Ironically considering the subject matter of the film, you can't replicate that kind of stuff. Multiple versions of the movie exist because of the politics of leaving the ending too open ended. I personally prefer the Final Cut, with my first experience being the original directors cut. But another part of what made it so cool was the multiple versions were more than just alternate endings, they were literally different interpretations each with it's own separate feel of exploring either him being a human, him being a replicant, or leaving it a big question.
Point is a lot of this stuff happened without planning it, and they won't be able to do it again. It will be forced, probably with too many corny inside references to the original. They'll try too hard this time and make it terrible.