Prometheus didn't need to be a prequel of Alien but its own stuff in the Alien universe.
Prometheus and its following didn't need to lead to the first Alien movie but could have just focused on the Engineers,their culture,their ways and their tie to the xenomorph.
In fact i would have prefered that Prometheus was closer to a story like in the mountain of madness from lovecraft or forbiden planet ! About an ancient race which try to play gods and gets wiped out by their creation which happen to still haunt the remaining of their civilisation.
This said, Ridley should have kept to his initial vision and not try to give people f..king Aliens as those so called Aliens are one of the main problem with Covenant !
At this point the best Ridley can do is close the circle with a last movie by finishing the David arc and stop making Alien movie or i fear that he will kill the franchise once and for all.
Regarding the xenomorph, i think that it is not cooked. The only thing which is cooked is the aspect of its lifecycle which has been rehashed since the first movie: Egg-> Facehugger-> chestbuster-> Xenomorph. This is boring and doesn't bring nothing new to the franchise. At least the movies Aliens and Alien 3 try to bring something new.
I think that a new Alien movie should make the economy of facehugging and chestbursting and try to focus on some aspect of xenomorph never seen before while still being consistent with the universe.
I would personnaly like to see:
* A single xenomorph either sentient or which did evolve to something way more sinister,smart and dangerous. As the Xenomorph is an alien creature, an inovative writer can develop other aspects of the Xenomorph never seen in a movie. Evolution toward sentience and/or continuous morphing to become more dangerous and adapted to its environment at each iteration would be amazing. How some human capable and SMART face such a foe ? Do they get wiped out or do they manage to make it run for its money ?
* A massive/overwhelming hive of xenomorph fighting mankind in all about war and winning despite man technology. Some exotic types of xenomorph could be introduced resulting from mutations/evolutions achieved in order to fight against marines weapon