I don't think the murals proved anything other than the Deacon being their ultimate creation with themselves, the engineers, as the basis to obtain it previous trilobite facehugg
The rest of things they obtain are direct black goo mutations.
I can't really see they solving the terrible mess Prometheus was. All just because they wanted to remove anything alien related from that film (to the point of even changing everything related to the jockeys, ship interiors and tech included), and now they want to come back to it.
They even removed the original Fitfield scene where he was probably turning into something, if not the same, similar to these neomorphs we now know they are obtained by direct black goo infection on human-like beings, this including engineers not just humans. They opted for that zombie thing, and now it becomes another problem that they will just have no way to deal with it other than embarrasing retconning, hypothetical Alien 5 style.
They were embarrased of the alien in prometheus (Scott's talking that he had seen it at Disneyworld, etc). Now they will have to retcon in covenant.
And all other stuff that was left unexplanied in prometheus, such as the green crystal, etc.
What a mess.
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By the way, and being obvious the alien we see in covenant still isn't the biomechanical one..., one of the things Weyland used to say to David, is that he would never had a soul because he wasn't a man, or in other words, a living being. In the end David could see the black goo as a thing to become somehow alive, and might end up using it on him or walter. However, if the classic alien resembles the engineer tech, their suits I mean..., wouldn't that be what he is eventually going to do?, to fuse engineer suit tech (or general engineer tech) on this new alien of his he has just created, and not on a synth like itself?
I think the origin of the classic alien will go that route. A fusion of what David has already developed with engineer tech