I'd like to add two threesomes - Axel/Lena/Ian, and Lena/Axel/Agnes. EDIT: Also Lena/Ian/Ivy.
If Axel and Ian end up becoming good mates, and providing Ian tells Axel about his having had sex with Lena when it comes up at Shine, I feel like a threesome between them is not that far-fetched. It would need to start quite a long way along in the game though, because I feel like Axel has to get used to the idea that he doesn't own Lena and she's allowed to do her own thing, first.
As for the other one, it's pretty obvious that Agnes (she's the mature fashion designer that sits in on Seymour's photo-shoot with Axel and Lena) has a thing for Axel. If they're gonna go anywhere with that, I feel like it would include Lena. That being said, I feel like Agnes would take more of domme role in the situation and use it to direct Lena into a supreme submissive situation (gagged, blindfolded, maybe hogtied) with Axel being the dom just under her; as in, Agnes will then have him give her pleasure once Lena has been pounded into an incoherent mess. I can imagine that happening if Lena follows the submissive path, and has a really high lust score. Agnes just strikes me as being a dominatrix with a lot of boy toys (like a female Hugh Hefner lol). Doesn't help that the characters is based off of Miranda Priestly (Meryl Streep) from The Devil Wears Prada.
I also feel like Ian/Cindy/Axel is a pretty obvious one. This could either happen after Cindy and Ian end up in a romantic relationship (if he chose to kiss her rather then fuck her during Wade's birthday) and Ian decides to grant a Cindy who's thoroughly embraced her outgoing nature the chance to fulfill a fantasy (Axel fucking her). Providing Ian and Axel, or the three of them, are friends. Or maybe the more obvious/likely 'corrupted' path of Ian and Cindy fucking behind Wade's back and the more daring aspect of her personality emerging as a result, allowing her to voice her desire for a threesome with Ian and Axel during a photo-shoot or something. Both of these would take some time to come about though, seeing as Cindy needs to get over Wade, or her sense of guilt, enough first.
There's also the possibility of Lena/Ian/Ivy, which I also don't think is that far-fetched if Lena becomes more sexually liberated (so she doesn't associate sharing Ian with her bad memories of Axel and Cherry), and Ivy develops enough of an interest in Ian from her meetings with him and what Lena tells her about Ian. And of course, that Lena and Ivy's relationship remains tight.