1.) Nancy has already confirmed she doesn't mind sharing. Plus, she was already in the game's first threesome with Nova. Also, she seems to be into fantasizing with group sex with the MC and her daughters. Not sure if this will happen, but to be honest... Cari has HEAVILY hinted this is a possibility with Nancy's dialogue. Not sure if this happens if it would be treated as incest; maybe as long as Nancy and her daughters don't like... sexually interact with each other, it wouldn't count. I dunno. This one is pure speculation on my part, but hey... a guy can dream.
2.) A Luna/Annie threesome scene is pretty much guaranteed. For anyone paying attention, it is pretty obvious that is what Luna saw when she accidentally touches Annie. It's even further heavily hinted at with Annie and Luna's dialogue before Annie's restaurant date with the MC.
3.) A Dalia/Alex threesome scene is also pretty much guaranteed. Even though it starts friendly, Alex and Dalia do flirt with each other a bit throughout the game, and Alex and Dalia both hint at interest in a threesome (think Alex brings it up to Dalia when she's in the game, and I know Dalia brings it up in passing somewhere). So, very safe bet of a threesome here. Plus, it looks like sexypeanut confirms this, and they're a very reliable source of information.
4.) I can see a Nova/Penelope scene in the future. Nothing really hinted at here, but they both like each other, they both have a crush on the MC (that both have acted on already at this point), so I can see them sharing/getting involved with each other. Again, this one is speculation on my part, but it wouldn't be unrealistic. EDIT: Shit, I totally lied here. I just got back to the college party Nova, Penny, and the MC go to, and it is HEAVILY hinted at that Nova and Penny are crushing with each other. Can definitely see a threesome scene here.
I can't really think of any others at this point. But with the flow of the story, I can see everyone's comfort level with each other building to the point where all relationships come out in the open with success, and the sharing builds to foursomes +, so we'll have to see.
Personally, I want more Calypso content. This next update, considering where the last one left off, looks to deliver heavily on this!