I understand your concerns, Zeta Reticuli.
I hope that the movie won't disappoint you, or not too much.
For my part I'm glad that Scott has decided to do something bigger and to tackle bigger themes (if only by showing the way). And I find it appropriate in so far as I think the mystery of the SJ from Alien let the door opened to that direction. Furthermore, Scott has repeated that he wanted to do something different, even though the film will share many things with Alien, notably at the end, if I read him well.
In the meantime I think that it will not, if well made, ruin the movie to your eyes, because it's Scott that makes this film. It won't turn into a Roland Emmerich's blockburster. I mean : Stargate is cool. The pyramids that are used by spaceships, that's good, even though it is as far-fetched and not-to-be-taken-seriously as the first AVP plot. But it's just a basis without exploration (because of the nature of the film) : Ra is our god, and that's nearly all. Conversely, Prometheus seems to be a movie that will give a meaning to its premise - and not the "creationistic" aspect only, of course. There will be provocative aspects. Sure, all of them will not be built upon 100% solid foundations, but it's nearly always the case in SF.
Frankly, if I have fear for the movie, it is not for this premise of a role of aliens in human evolution (in what way, we will see). I just hope that it will indeed be provocative. And I think it will, because Scott would not have said what he said in interviews if it were just a gory Avatar.
I think that when you allude to the "fantasy" aspect, you are exactly on the point that I evoked in my first post, and that produces topics like this one about many films or series (I remember that kind of topics about Caprica and Stargate Universe, for example).