Huh, I guess I haven't actually said anything about it here. Such an oversight on my part, sorry about that.
I hold firmly to the hope that the Main Girl Harem (MGH for short) is possible. Its conditions are bound to be intricate, but I already have some definite ideas about them.
Firstly I believe it will be an all or nothing situation. All the girls bring something to the table that is crucial to things working out. No combinations of two or three Main Girl Paths; you must either combine all four or stick to just one. So in a sense there wouldn't be a Bella + Jill throuple as they would be in a six person relationship or "sexouple" as I like to call it.
What precisely do the paths contribute? I'll give you some examples.
Bella is in the process of moving on from a failed marriage. I think it safe to assume that while it is possible that James didn't really love her, Bella genuinely loved him and a part of her still does. Sage has been in more than one relationship before, but I don't think it has ever been really serious and the same applies to the other three. Only Bella has experienced how actual love can fail. Without her the others will jump to the conclusion that the MC doesn't actually love them and that is why things didn't work out. But in order for Bella to bring this to the table you must be on her path in season 3.
Josy and Maya actually tried to share him and it was working. They have experience of the concept working in practice and the MC respecting their feelings for each other instead of being a greedy self-centred douche. Without this input the others won't have faith in a multi-person relationship. But if you didn't choose to be with them in episode 4 and manage a decent balance until episode 8, they can't bring this input to the table. And crucially being on Bella's path in season 3 does not negate this.
Jill's relationship with the MC is much tamer than the others'. She knows that he isn't just after sex. Even Bella let things get to at least foreplay level. Jill knows that the MC isn't just after sex, but actually cares about the purely emotional side. Even without sex he wanted to be with her. But in order for Jill herself to believe this you must get the episode 9 bench conversation right and that can only be done on Bella's path.
However, Bella, Josy and Maya might just think Jill was being naive unless they hear what Sage has to say about it. With Sage and only with Sage the MC pushes for more than just being fuck buddies. He is the one who demands the emotional side in their relationship. I'm undecided on if this comes across better if you friend-zone Sage until episode 7 or if you get into the fuck buddy relationship and it may be that either one is okay. However, the MC must tell her about Chad and then spend the night keeping his hands to himself which can only be done if her path is unlocked at episode 7 or earlier. And once again being on Bella's path in season 3 does not cancel this out.
So you need both Sage and Jill for this. Without Jill the others believe sex is still a requirement and without Sage they won't take Jill seriously or believe that the MC will pursue the emotion even when he gets sex. And even if Sage is believable by herself, Bella won't consider the idea unless she has Jill's input.
One more potential condition might be that Sage must be at Bella's on Thanksgiving instead of coming to Maya's parents place and so you must NOT tell Josy about Chad. That way all four paths are at Bella's and have a lot of time to discuss things without the MC there. Another reason this has to happen on Bella's path is that otherwise the TV won't be set to the porn channel and the conversation won't get started without the shocking revelation. Note that if you chose Bella and broke up with Josy and Maya, Maya will learn about the relationship by default. And of course you have to get the documents or the harem won't be impressed enough. Impressed enough to suggest the six-way relationship that is. Impressed enough to send Derek away and have a sixsome then and there? Probably too much to ask, but I dare to dream.
Messing around with Side Girls is probably not allowed, but your past with Zoey is canon for all paths so her joining might still be okay, though it would be a seven person relationship at that point, so I'd lose the sexouple pun. And I'm just going to have to put up with that. It's a reasonable price for perfection.
Season 3 major choices will probably have significance so you might have to have Neutral affinity for this (and certain specific choices from seasons 1 and 2).
Like I said, I certainly have hope for this, but even if it will never be part of the game I have solid fanfiction ideas and I'm damn well writing it if DPC won't include it in the game. I'll only be able to do the story itself though. If people want visuals, other people will have to contribute. Volunteers?
