Fourth stage.

That's a very interesting idea to consider pulserifle187!  The filmmakers have indeed talked about something along those lines in the past: the thought that the Aliens could possibly continue developing towards a more civilized form. The concept is different from the alternate ending where the Alien rips off Ripley's head and suddenly starts speaking in her voice. Instead, the alien advances to its final stage of maturity gradually, then perhaps lives a long "scholarly" life after reaching the final stage: possibly becoming an intelligent species (in some unused ideas that were looked at).  So, it doesn't remove Ripley's head to acquire her intelligence or begin speaking english immediately:  her IQ doesn't drop while away.

The lifecycle might be much more complicated than we know.  There's already a bunch of stuff going on in Prometheus before the facehugger stage that helps explain the early development of the creature (but it doesn't link up with the classic Alien yet).  It all depends on what Ridley and the others have decided to go with.  The life cycle would be cyclical:  the creators don't build in straight lines, so why should Ridley? 

I think the full lifecylce we're dealing with includes a couple stages that we haven't seen yet, and imo what you suggest is definitely plausible. The only problem is that the Engineers and Aliens might then technically create themselves in an endless loop.  But it's only a problem if you view it as such.  The logo with the Alien eating its own tail could even symbolically represent its immortal and eternal nature...

Ridley once stated that he felt the Alien in the orginal movie was dying.  I believe that the Marines were looking at it all wrong in ALIENS: that it's incorrect to compare part of the lifecycle to ants.  There were ideas about the Alien lifecycle being somewhat similar to a parasitic Wasp/insect species when the first movie was developed, which may be where some of the coccoon ideas come from. Yet the creature's whole lifecycle may be harder to explain.  Conceiving them to be similar to Ants never entered the picture until ALIENS.  The early stages appear to be more similar to a parasitic wasp species that uses spores to turn Ants into mindless zombies (like Fifield) until baby wasps eventually burst from their heads...   

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I would be happy to see Ridley bring back the unused idea (filmed, but cut due to pacing) that the xeno cocoon's people and they morph into an egg, housing a xeno-dna-based facehugger.

Deleted Scene

It all depends on www.ourcase.co.uk what Ridley and the others have decided to go with www.casesam.co.uk.

I even wondered if the Engineer and their towering form is the result of tampering with a humans DNA, siting Shaws comment: "They are us..." or something like that. The Engineers are connected to us as how we see ourselve as Humans. If it goes right you result in the chalk-grey/white Engineer presence but if proper guidlines are not in place for gradual mutated growth and assimilation of said DNA then you get a runaway Fiflied effect/reaction....

Well there was the idea of 3 Stages... 4th Stage is Egg Morph and the idea was that Victims are morphed into more Eggs to start the process over again, this is based off Star Beast Original Ideas and Concepts, and they did try to go that route for Alien... but the scene while it was supposed to show Bret and Dallas at various stages of Egg Morph, it looked like Dallas was Cocooned so that the Bret Egg, Face Huger or what ever would then have a Host.. but nope the idea was they would both become Eggs..

As for the 3rd Stage, the idea was that the Xeno is a Child still at Adult Stage, it had nothing to interact with as far as its own kind and so only kept its animal/survival instincts... the idea of the Creature as far as Star Beast was that as they reach this Stage they can then be taught by its own kind to become more civilised so to speak.....

i guess the point is if you left a Todler on a island and made sure it had food and drink but it never saw you or had any interaction and when it was old enough it was left to fend for itself... by the time the Child Reached say 20 years old if some Travelers came to the Island and especially (not being Racist) but they was of a different skin tone.... they would not recognise the Travelers and maybe would act Hostile and Savage towards them.... or indeed any Human as it would never had seen another of its kind.

So the idea was that the Creature (Star Beast) would Mentally Evolve and could be taught by its own kind to then be civilised and that the Organisms had some pretty Ritualistic Civilization