To be honest, I'm still hopeful that the original ALIEN movie lifeform...which was a tough sonofabitch with acid for blood...turns out to be something that had 'evolved naturally' that way over time, due to some extremely harsh conditions on it's unknown birthplace.
Having disappointingly 'retconned' Giger's fossilized 'spacejocky' skeleton into a tall, bald humanoid strapped/helmeted into a navigation device...I was hoping that Ridley's prequels would still keep the origins of the original 'xenomorph' creature as being of a completely natural, mysterious nature...even though PROMETHEUS kinda hinted that this organism *might* have ended up being the result of some past 'bio-toxin' ('black goo') experimentation by the 'Engineer' race...
(well that's what the movie implies to me at least, seeing as we see a 'xenomorph'-looking carving on the wall of the 'ampule/giant head' room)
Thing is, Ridley initially intended to move away from ALIEN's original 'xenomorph' design I believe, and seemed to be attempting to give us 'different'-looking creatures instead...so I was hoping that the 'black goo' material was actually some kind of 'by-product' substance from the 'xenomorph's world which the 'Engineer's had come across during their past travels - a substance which they gathered in quantity, as it had some 'transformative' properties which were of great interest to them.
However, it seems from early reports that the 'black goo' is affecting parts of the 'Engineer's homeworld in ALIEN:COVENANT...although it remains to be seen if it's actually a 'naturally-occurring' substance on that planet, or not...
Personally, I'm still hoping against hope that the 'black goo' on 'Paradise' turns out to be the contents of some 'urns/'bio-weapons'? which were onboard the 'juggernaut' which 'David' and 'Shaw' took off in...and that David emptied some of them out onto the ground to see what happens, which resulted in the indigenous plantlife forming the deadly 'spores' mentioned in recent reports...
(this would allow for the 'black goo' stuff to remain an 'alien substance' from the 'xenomorph's homeworld, rather than something which originates on the 'Engineer's homeworld itself)
Unfortunately for me, it *may* turn out that the origin of the ALIEN 'xenomorph' will be 'retconned' in ALIEN:COVENANT to be the result of some long past 'transformational' experimentation caused by the 'Engineer' race, using naturally-occurring 'black goo' from their 'Paradise' homeworld...
...and given that Ridley may *still* wish to veer away from giving us the exact same look of the original 'bio-mechanical xenomorph', we could just see similar experimentation by 'David' giving a range of 'different'-looking results - the 'leaked' pics of 'albino'-colored eggs etc. certainly seem to support this notion...
(I'd actually be quite happy with this outcome, as the long-crashed 'derelict'/'juggernaut' eventually seen in ALIEN could just be implied to be carrying an indigenous species from where the 'black goo' substance originated...rather than a creature specifically 'created' by the 'Engineers' through 'experimentation of their own!)
On the other hand, I could always stick to my preferred view of the ALIEN movie if things don't turn out in a way that I like...and just look on these prequels as being and 'alternative universe' version of events...