The way I see it, this VN will consist of three acts:
1. the Lilith act
2. the Abby act
3. the Becca act
What we know so far:
Each act will be supported by a side story (with or without aliteration as the title, still to be seen here with Abby) that has the eponymous character as the main character.
We gain deeper insight into the character's mind and her relationship with Frank, respectively how it develops.
Based on our experience with the first act, we can draw the following conclusions:
Act 1 actually had two major dramas:
Abby learning that Frank is not her biological father and the changing perspective of her towards Frank.
The breakup between Lilith and her boyfriend and her acceptance of Frank as her father was the second drama
What dramas can we expect to see in Act 2, if any? The oft-mentioned appearance of Abby's father will probably play a big role. I think he will also use his wife's presence to lure Abby in with the perspective of a mother figure. While we don't have to fear that Abby will be baited by this, I would imagine it will come down to a team-up between Abby's birth father and Shannon in Act 3.
Let's move on then to Act 3, the Becca act. Unlike Abby and Lilli, she's pretty into Frank from the start, and without making a big secret of it. As I write this, the following train of thought occurred to me:
What if the whole "Becca could be Frank's daughter?" was a red herring by CNG to throw us off the scent? Here's the scenario I could imagine as the big drama for Act 3:
Shannon finds out that her daughters live in the same house as Frank and Abby.
She may also find out that Frank has something going on with them.
While she can't do anything to piss Frank off about Lilli, the topic has been discussed extensively here in the thread, so she could use the mystery of Becca's paternity to sabotage Frank's life. Sort of along the lines of "By the way, Rebecca is your daughter, I just never told you because we were married a long time and you were around long enough."
Even if this is not true and the truth comes out, this will probably result in a small catastrophe for Frank's professional life, possibly also with the intention that friends and allies of Frank turn away from him.