After I saw Prometheus, I mused a bit on how I would have improved it. As that isn't entirely a fruitless endeavor, especially viewed through the lens of my story setting, I figured I'd follow it a little ways. Give it some rope. I still say that doesn't mean Prometheus was any good; I look at it more as a failure that is like a cut in your mouth. You can feel it so you keep touching it with your tongue, even though it hurts. So here goes: the Engineers are H.R. Giger people. That is the core premise. Despite being bipedal & possessing thumbs, they aren't humans; they are aliens. The scientific thrust of their civilization is biotechnology. They've applied it to themselves; the ridiculous bodybuilder muscle mass is an obvious visual cue. It wouldn't be too hard to program a body not to reabsorb extra muscle mass, & in a post-scarcity world that might be what the Engineers did. They are bigger, but within a margin of error, within plausible limits of surface area to volume proportions.
The Engineers-- or Prometheans, if you are feeling the muse-- find a world & seed it with a sacrificial death. One of them drinks the cup of black oil & dissolves, scattering the bits & pieces of organic life on the planet. This sets in motion the possibility of carbon based life, of chemical compatibility. I assume they have to find the planets, rather than terraform them. How would they terraform it? That is a pretty great question, actually. Maybe they tow it into a better orbit with spacewhales? From there they could reasonably scatter biological machines to you know, convert the various atmospheric poisons into the atmosphere they prefer to get oxygen for energy transfer & condense the water, since that is the solvent they use. Except, well, if they have enough biomass to tow planets & biomechanical terraforming nanites, how much more do they think they'll scrape off the planet? Why not just eat it, digest the stuff you need? Maybe the spacewhales are also terraforming plants, & rather than a bacterial model the things work like factories?
That isn't really in keeping with the "seed & forget" picture that we have painted for us in Prometheus. The 2001: A Space Odyssey school of thought-- "we had a monolith uplift the apes, then put one on the moon to see if it worked & then one around Jupiter to see if they were up to the challenge"-- also fails when you note the reoccurring "giant pointing at a star map" architectural motif; there had to be Promethean intervention & contact throughout history. They've been showing up for a while, so maybe they actually were taking an active hand? Heck, maybe they chucked the asteroid that killed the dinosaurs off into the Earth. "Crap, these crazy giant birdlizards are everywhere, we need to clean out some niches for the mammals." Oh, possible entry point for Predator? "We made these other aliens to go to the planets we built & creatively cull them." Okay, maybe you don't need to mix your Predators with your xenomorphs & your Engineers, but I like to. Heck, I'd add in Blade Runner & Terminator if you let me, one big cosmology of cool science-fiction-horror films.
Once the planet is seeded with organic life, then the Engineers send the eggs. The facehuggers. The idea being that once the planet is chemically suitable-- carbon based life that uses DNA-- you send down the xenomorphs to organize that life into biologically suitable forms. You saw the suit that the Promethean wore; it didn't look like Aliens by accident; that is a xenomorph, biologically engineered to be worn like spacesuit. I say it is a living entity in its own right. Those "throat-like" tunnels that the xenomorphs live in? Engineer houses. They just ride around on aliens like they are horses, use the drones as workers, so on & so forth. The chestburster that came out of the Space Jockey could be an accident, or heck, it could have been a sacrificial offering like the first seeding; maybe an impregnated Engineer is what begins the second stage of "terraforming." Anyhow, the point is: those guys just love the crap out of xenomorphs. Sleep in pods. Dream with the hive mind. Pure bliss. Utopia, honest.