By far the most important aspect for me is the NTR. I still believe a good approach to it is that the MC being enough of a horny pig will have Alice get off other men just to spite him, even going as far as to lose her virginity to someone else in the heat of the moment at the extreme end of things.
What would feel the most like something marune would have done in the original game would be something like having a male character you hire. You get Alice drunk on a date and try pushing yourself on her, but she isn't up for it and decides she needs to go back to her room, but she sure won't trust you to take her to bed, you dirty man, so she calls up your security guy to take her.
But he's just as much of a horndog, so you follow them and find him coming onto her in her bedroom, with the same array of actions as the other NTR scenes. [Watch] [Say Something] [Attack Him]
And then you can watch his woo-ing work on her as they soon get down and dirty and he takes her virginity while you just jerk off outside the door.
That feels like something the original would have done. More date-rapey. Less to my tastes, but good enough.
NTR for the female character would take more thought, since the ex-army security guy had nothing going for him. Could still write Alice as a legit girlfriend prospect for the player, but I lean straight more, personally. I'm open to it, but the original made it more like a pushy, licentious friend trying to groom Alice to getting into the raunchier side of the work, and not like they had romantic chemistry. If the player could be coming onto Alice and still lightly pushing to sexy her up, but ceded to her hesitancy by flirting about keeping Alice to herself, there's some potential there. Can have their relationship lean harder over time toward "Men are much filthier and rougher than women, I don't blame you at all," and angle it like that. Because I'm not a man, you don't have to worry... That kind of thing.
Can do NTR with Julia or still go for men, with the triggering point being after they're a more official couple Alice starts taking umbrage with the player still sleeping around with men. She's debasing herself! Fucking low-life guys that don't even respect her... The business is doing well enough that she can stop! Yet, of course, the player loves dick. Who doesn't? And they explain to Alice that it's not just good for business, its just plain good. Alice is sneers at this, "Doesn't our relationship mean something to you?"
She storms off to stew on it. This plants it in her head that the woman she's somehow gained a fondness for loves dick, just loves being used like a whore, and Alice will be curious what it's really like... So eventually, she'll give in to temptation and try it.
it'd have a very different feel to it than the male player one, but works, and I think can work very well.
And I don't think the original game could have ever done any female player NTR like that, in any form. I think it'd just be all about grooming Alice to be as much of a whore as yourself.
You're welcome to hire me as a writer.