NTR wouldn't fit the plot in my opinion, and I don't think it would really fit Becca's plans either. I remember vaguely that there were people who pushed Lilith's longing a bit in that direction, either because of the scenes between Lili and her then-boyfriend or because of the scene with Abby and Lili (I'm not sure about that, but most people got upset because of the scenes with Lili and her now ex-boyfriend and I just think to myself: Jesus, the girl is 19 and it's mentioned during the plot of the main game that she already had some boyfriends. I would be much surprised if she was still a virgin after x boyfriends).
I think it will be more towards light harem, Becca already said that her target is "Frank, Abby, herself, Lili and more people Frank wants in this relationship". A favorite for me would be Meiko: Her parents want to set her up with Frank, she obviously has feelings for Frank and Abby, Lili and Becca like her, so I think she will join at some point.
As for Lili and Abby: Lili is very protective of Abby and the only realistic situation for me of Lili being okay with Abby having a thing with Frank is the following: Frank and Lili are at it and a drunk (or at least pretending to be) Abby joins them. Ideally, Lili would already be very much in the mood in that situation, so Abby would have a very easy time "mouthing off" to Lili so she could join in. However, I don't see that happening for now, the focus will be on Lili building up her self-esteem, since you can learn that she feels a little intimidated by Becca and Abby in terms of her physical development. Therefore, I also think that a too early scene with Frank+Lili+Abby would not be helpful for Lili, as she would then possibly see herself in competition with Abby in terms of attention.
As for the lesbian couple Vanessa and Elizabeth: Elizabeth's bio says she wants to have a child and that Vanessa isn't against it, but she thinks they aren't ready for it yet. I doubt that they will be part of the quartet (Frank+Abby+Lili+Becca) or quintet (+Meiko), but I can imagine that if Frank respects their wishes, he might be asked if he wants to be the father of the child.
It is mentioned at the beginning that more characters will be added, but I think that the current characters are already enough as important people.
So in short I can say Becca's plan is Frank+Abby+Lili+ herself plus optionally Meiko. Vanessa will rather stay out of it, I think. Her wife Elizabeth is more bisexual but I think the two of them (her and Frank) are leaning more towards friends with benefits and only if Vanessa gives the green light, she knows about it and Frank plays by the rules.
At this point, another interesting assumption that was voiced and that is quite plausible (but so far without evidence for and against):
Becca is Frank's biological daughter. Shannon and Frank were a couple for a while at 16, until Shannon had to move in with her aunt for unknown reasons. Shannon is currently 39 and Becca 23, meaning she had Becca at 16. Shannon did not meet and marry Lilith's father until a few years later. That means it will come down to one of these theses:
1. Frank is the father and Shannon had to move away because she was pregnant.
2. Frank is not the father. Shannon had an affair/one-night stand with Becca's biological father at some point.
3. Becca is not the father, Shannon did not have an affair when she was with Frank but got someone new relatively quickly who got her pregnant.