Gonna repost this from another thread.
Rewatched Prometheus. I think it's clear with the mural and Deacon at the end that the black goo can ultimately evolve into some variation of a xenomorphic creature, the variation of which depends on the DNA involved. Both the Deacon and the Neomorphs are examples of this.
What David has done is add in specific genetic material (space wasps, Shaw's eggs presumably) over a long period of experimentation to create the traditional Xenomorph we know and love, ultimately leading to the Queen and everything. And you know what, that's kind of an interesting story, even if it's not what I would have preferred. David is clearly the main character, and following the story of the creator of the Alien is interesting. I can appreciate it for what it is. The Alien is still mostly the creation of the Engineers, David is trying to perfect it. It also allows for new variations in future films, which should be cool. I'm open minded for now.
And no, you simply can't discount Prometheus. A:C is a direct sequel to that. And when you think about the creatures and what the black goo did to people in that movie, along with the mural and the Deacon, it clearly points to a specific lifecycle (egg (Shaw's womb), facehugger (Trilobite), chestburster (Deacon)) and creature that ultimately comes about.