Well you are always going to have difficulty being knocked off your feet by a film the older and more stuff you see. The trouble being not much original gets made, they are all pitched and financed as something else in oder to get the money and dickhead studios to back them, Alien itself was pitched as Jaws in space. Its never going to be a trully original idea, its based on films that already exist, plus Alien itself wasnt original, O'Bannon said he didnt steal from anyone, he stole from everyone.
Actually I dont expect it to be an Alien film, the average joe punter might, but might not be as informed as me, I have been quite aware that it started out as that, but evolved into something else, into a wider story in the universe telling a story about the other aliens. If you take that into account it wont be so bad.
I'm hoping its the best 'Alien universe' story it can be, rather than looking for it to be super original. Its not a matter of how original it will be anyway given the source and history, but its what it does with all the elements that inform its creation. I think Scott is a great enough director to make something trully brilliant, bringing all the elements together into something, if not exactly original then a fresh and exciting view on old material.