If MP keeps adding paths and characters and genres, the game/story will fall into shambles (just like AWAM). AWAM attempts to be all things to all people and have added way too may characters, way to many kinks/genres, and is a complete disaster. I can only hope MP is smarter than that.
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I'm just going to mention novels from JRR Martin and the HBO series "Game of Thrones"
Characters come and go... the Novels from Martin have new characters being added (and subtracted) while the HBO series didn't have a lot of new characters...those that were added often stuck around till their utility was done with.
So it's not a matter of adding characters and some plots between them and the MCs, it's the story's main themes and plots that matters. So if MP adds Anna's mother does it change the game? No, not really. But you can use the "mother" to help define Anna's character, her past, and something that explains Anna's motives {I really don't expect to see the mom unless it's a plot fix}.
Adding Marcel which isn't too late to do so can add to the game in many ways and even change some dynamics. Just as an example imagine Anna has choice (A) going to dinner with Jake or Choice (B) going to work out at the gym (Marcel). Where Marcel is total irrelevant is adding him and BBC way late in the game.
It's just me, but I don't think Mr. Palmer is going to make Anna a Hose-Monster Slut who stops being a lawyer and becomes a porn star or something like that. Everything is being setup it seems that that the males act on Anna and that it gives Anna the chance and excuse to engage in some naughty. My reasoning is simple. If Anna takes a lover knowingly and willingly, it's pretty much game over for the marriage, but the game has two outcomes that come to a conclusion--the marriage is saved, or it isn't. From my perspective till the ending is reached there will always be a chance to save the marriage. So if Anna takes Jake as a lover and how Anna's character is portrayed, its over for poor David.
But if throughout the game one set of seduction after another and Anna's own desires and lust allowing things to happen, the game's premise is maintained... just that eventually everything comes to a head and an ending is reached for good or bad