The black goo breaks the DNA structure of an organism to make way for new DNA structures. David said "To create, one must first destroy." This was shown at the beginning of Prometheus. The Engineer ingests the goo and his DNA breaks and new life comes out of it.
So you guys are wondering what does the black goo have to do with the Xeno? This leads to another question in A:C. Why did David nuke the Engineers? The answer is simple...
David wanted to create new life from Engineer DNA. By nuking them, he breaks their DNA structures to make way for new DNA structures. Guess what are the new DNA structures? Aliens.
^ Now, look at the interesting poster with 3 heads? From top to bottom, an Engineer devolving into an Alien. What this means is that the Xenomorphs are the ancient ancestors of the Engineers. This also means that the black goo does not actually create "new" dna but it creates OLD ones. It regresses the evolutionary cycle of the Engineer and their most primitive form is the Xenomorph. This is why the Engineers in Prometheus pay homage to their ancient Emperor in the form of the mural, the Ultramorph.
The same thing happened when the mohawk scientist in prometheus ingested the black goo. He turned into a sub-xeno humanoid.
Looks like the black goo reverses our genetic structure.
Engineers > Humans/other Engineers/Yautjas > Protomorph > Mohawk Scientist Alien > Neomorph/Deacon
The modern Xeno is most likely at the Protomorph stage with synthetic characteristics thanks to David.
So there you have it fellow forumites! The ultimate question has been answered. No need to wait for the next few years for another Alien movie when David ingests the black goo and turns into a giant synthetic engineer with a cargo full of biomech xeno eggs heading to earth but before he gets there, he gets chestbursted and thats when Ripley and her crew encounter the King's corpse.