A couple ideas involving that suit Rebecca's wearing.
First, being like an octopus, what if the tentacle alien were to somehow get into her suit, where you'd see his outline traveling up and down around her body as she tries to get a hold of him?
Second idea might not be possible with the animation models, but when it comes to tight suits like bloomers or bike shorts, it's always neat to see the camel toe manipulated through the fabric, like the tentacles causing the pussy to gape from outside the clothes, with sort of a skin tight outline.
Finally an idea for when the alien goes all the way. I'm thinking of a setting similar to the lead up to the first sex scene from the movie Sliver, starring Sharon Stone. It's hard to find the whole scene, but it starts with innocent flirting, with the guy admiring her ass as she works out. Then he invites her back to his place for a beer, and shows her a volcano sculpture, and while he distracts her with some strange talk about wanting to fly into one (a volcano) he forcibly pushes his hand between her legs and starts masturbating her through the crotch of her leotard. From there they roll around on a couch until he kisses his way down her body as he pulls down her leotard, till you see her naked hips and he goes in for an off camera pussy kiss. Best part comes as the scene dissolves to her looking down and moaning as if in distress before the camera pulls back to reveal her naked ass straddling and grinding on top of him, in a position similar to Enigma's with that dinosaur thing. The scene can be found muted on Xvideos under "Sharon Stone In Sliver Clip 2" It just looks like a sexy sort of reluctant/forced sex scene.