The Alien can’t be the main thrust of the story because, well, with the exception of Ripley and Jones, no-one survives an encounter.
So you need a story of intrigue which has the alien as the kick off or the denouement. Like in the Expanse where the fungus thing is its own plot.
Maybe the characters are a cult extraction team (people who remove rich kids from the ensnarement of a religious cult). They are shown a video from a billionaire’s kid where the youngster says they’re going to evolve. The patron wants their kid back …dead or alive.
They track down the cult through various dead ends and then start getting reports that the people they’re questioning end up dead. They get apprehended but while in custody, someone else is killed. They’re just about to get trackers fitted when the police techies discover they’re already being tracked!
Eventually they arrive at the cult headquarters, accompanied by a police presence (walking, talking bullet sponges) and it’s eerie quiet. They find empty egg sacs. But no sign of the cult…
…and of course…the players have to role play not much knowledge of what’s going on…
The cult, they determine have escaped off world to a nearby system. But attempts to contact them have ceased. The police just want to leave it but the patron says “dead or alive…” and reveals that they’ve definitely been murdering anyone on the way through shady connections with private security.
The players then have to track a starship which is somewhere out in the void…potentially filled with dozens of aliens…in order to get their payoff (or to prevent being killed). To track down the corpse of a youngster.
So they decide to go find it. They discover it drifting in space and all comms are down. The aliens have gone into their dormant stage. Getting aboard and retrieving a corpse will be tricky…especially when they discover the kid is still alive! Totally traumatised but alive. The gear aboard is too damaged to do a medical scan. ….do they bring this typhoid mary back with them to Earth or wherever? Fire them into cold sleep and wash their hands of it?
Sure. of course they do.
And the first layer of the onion is revealed when the private security arrive and take the youngster into custody in a sleep pod. …