No Ripley,instead go for WY plowing ahead on the bio research on just the limited tech they have been able to gleem concerning the xeno`s.
Set after ALIENS,possibly before/after A3,or no A3 at all.
Some bright spark tries his hand at molecular bio engineering that has graive consiquences for that Off planet orbital Bio research facility (yea Marines in Space suits

from the tech manual).
Colonial Marines are called in to investigate what happened to the research facility. A full detachment (CO,Synthetic,2 dropship crews,20 Marines) of Colonial Marines is sent (Dropship to check out the unknown craft,Main ship & some Marines on overwatch of the whole area-station,the remaining squads floating with dropship escort,over to the facility).
At a similar moment in time (to the Bio-engineering work) an advanced species tracks readings of this bio experiment & thinking they are the Xeno`s they capture-kill,go after the readings at the facility.
Colonial Marines arrive after what looks like the facility has been assaulted & an unusual craft docked.
Plenty of destruction from acid & weapons damaged (unknown types) around & in the facility. They make their way to the Docking bay to secure for dropship & CO`s arrival.
Then the search truely begins of the facility,eventually leading to survivors.
Survivors do exist but in such a shook up way bar a small Tactical WY Security Unit (hardened former Colonial Marines). After a debrief from the WY Tactical Unit decided that not only is their a Bio engineered creature running amok,that also this advanced Species are too.
Colonial Marines find at cost that the CO is in fact an "Artificial Human" planted by WY to safeguard their Research work & facility at all costs to the Colonial Marines. Upon discovering that it`s Colonial Marines vs WY Tactical Unit & former CO.
The Colonial Marine Synth is actually on the Marines side due to correct programming & thus takes out the CO (maybe?? ).
The rest,well fire fights galore & as said Marines dropping as too surviving techs. Of course the advanced species take losses from both sides (Bio Creature & Colonial Marines).
It`s whilst this ensures that a captured xeno escapes the advanced species ship (due to some offset by the close proximity of the Dropship) & via connected airlock to the facility starts it`s own escapeds on the Humans,Advanced Species & Bio Creature.
Leading to an interesting ending???
Well just one idea to the one`s already offered.