Every once in a while I like to revisit my idea for an Alien sequel. This is just a rough outline.
A woman named Selene, formerly a data miner for the now-defunct Weyland-Yutani Corporation is pursued across the galaxy by a team of bounty hunters on the payroll of Charles Weyland. Before the company fell, she obtained and sold some of their secrets to Lasalle Bionational, WY's chief rival. One day her shuttle is bound for a farming colony on the Outer Veil, but it is intercepted by the mercenaries' ship, so Selene's shuttle's computer reroutes her to the nearest habitable world, the rocky planet of ZT-701.
When she awakens after the crash-landing, Selene knows she must hide from the mercenaries. She takes refuge in a set of ruined human structures. On further investigation, she finds signs labeled with the insignia of Lasalle Bionational. It seems to have been a science outpost. The entire complex is dilapidated and devoid of human presence for several years. The few isolated areas that have some intermittent power are those still connected to photovoltaic panels or geothermal steam wells. There is damage from both the natural elements but also from acid exposure, and there are a few scattered human bones. One of the walls has been painted with the words "KILL US".
But before she can enter deeper into the complex, the other ship arrives at ZT-701 and the armed mercenaries land on the surface. Selene thinks she sees something else flying around, a creature like an albatross or a hawk, disturbed by their arrival. The mercs scan the complex in infrared and the only sign of life they find is Selene's heat signature. She scrambles to evade her human pursuers but then encounters a horrific new kind of Xenomorph: the Cobra. She had no idea the Xenomorphs were actually real, and until now had believed they were just a fabrication by the company. This one is long, with no limbs, and it has a hood extending from its head which it flares out to both sides. Terrified, Selene runs the other way until she is captured by the mercs.
The merc captain Magnus offers Selene a clemency deal to return to work with Weyland's new company. In exchange she has to infiltrate the Lasalle complex alongside them to extract any remaining data from the outpost's central computer. Seeing no better options for her predicament, Selene agrees to go along.
As they travel into the complex, one of the mercs is ambushed by another new breed of Xenomorph: the Grizzly. It is more heavily-built than any variant previously encountered, and can stand up to firearms. Eventually the others reach an area of the complex which was apparently an animal preserve for many different species of imported Earth life, hinting at the purpose of this facility. In the "zoo" they see several open Xenomorph Eggs, and scattered bones from various animal species.
Before they can enter the core of the complex, they have to restore power to the security gate. During this diversion, Selene and a couple of other mercs encounter another new Xenomorph: the Lion. It is a quadrapedal variant with a mane of sharp barbs around its head, and a pair of long sabertooth-like fangs in its mouth. Selene deliberately trips one of the mercenaries and he is mauled by the Lion while she reactivates the power generator. The re-activation of the lights startles the Lion and it runs away into an overhead ventilation shaft.
With the remaining members of the team, Selene and Magnus enter the core and access the central computer. They discover that this facility was a testing ground for new breeds of Xenomorphs, operated by Lasalle's bioweapons division. Shortly before the Hadley's Hope Xenomorph takeover on LV-426, one of Lasalle's operatives stole several Facehuggers and smuggled them to the facility on ZT-701. They eventually broke containment.
But then the remaining mercs are swarmed by a group of Cobras. In the firefight, Magnus is sprayed with acid blood and loses an arm, where he is revealed to be a Synthetic. He and Selene are now the only remaining members of the expedition. They flee the complex for the landed ship where it is revealed that Charles Weyland was watching them all along. Armed with a rifle, he threatens to take the data Selene and Magnus have found for himself and breed new variants. But then from above he is picked up by the Bat, a flying variant of the Xenomorph. The Bat skewers him in mid-air and Selene and Magnus board the ship and take off from the planet's surface.
Together they set a new course, towards the Outer Veil and far away from the evils that brought them here...