I'm a NTR fan here , have played a lot of NTR games. One thing I haven't encountered is having a good ending on a NTR theme for obvious reasons. That being said, same as what you have written above, a good story version for instance, can have the MC losing out Christine to Sliver as he made the wrong choices. So after watching the yacht hot tub scene, he could try to win her back from Sliver . It wouldn't easy coz she has somewhat fallen in love but with good plot writing (such as using the webcam hack to expose sliver fucking other woman, not just Christine etc), it is definitely possible winning back from the bad guys.
So in short, you can create a guy that everyone hates, may lose out your gal with poor choices on alternative path, makes a 360 degree effect to chase her back and happy every after again. Which is quite a hollywood theme movie if you think about it. That would be my best suggestion if you plan to make future NTR scenes .
I do plan to do some NTR stuff at some point. I'm still thinking about how since I'm not personally a fan of that type of scene and the fact that I really don't like the Robert or Silver characters will make it harder for me as well.
I want to finish my hot tub and Spa events first though and at least initially there won't be any NTR there. I might go back and add some NTR for Robert to the hot tub at some point.
I plan to do a Yacht expansion as well but that will probably be fairly small. Was mostly just planning to have Sonya join in on the Truth or Dare game with MC, Jess, and Christine since at that point all restrictions as far as storyline have been lifted (you've already fucked Christine, and if you fucked either Jess or Sonya, she knows about it).
The problem for me when it comes to the Yacht, is that environment is absolutely huge and take so much memory that rendering on my computer takes forever. So I may not even do a Yacht expansion because of that. I may wait til they leave the Yacht and have them all party at MC's place or something instead. For comparison sake on the yacht it takes me about 2 hours to render each frame. If I did an identical scene in MC's hot tub I could render it in about 20 minutes. I think for an environment like the Yacht, I really need to upgrade my computer and I don't plan on doing that for at least another couple of years.
The progression I'm planning for now is for the hot tub to have some lesbo scenes between Christine and Jess, potential for BJ from both of them, potential for MC eating out either or both of them, potential for fucking Jess, potential for threesome, although hopefully done in a way that Christine doesn't know for sure that MC fucked Jess. Christine will not be fuckable yet, although dreams/fantasies are fair game for that.
For the Spa I'm planning lesbo action for Christine, Jess, and Sonya, MC fucking both Jess and Sonya, and threesome options with all of them. The blindfolds idea that the vanilla game introduced there should give me some interesting options as well for involving Christine without her knowing for sure who's doing what to preserve the vanilla game status that she doesn't know if MC fucked Jess til day 14. She already has the option to find out MC fucked Sonya in the vanilla game on day 11 so it doesn't need to be hidden in the spa.
Also planning to do at least 1 foursome option, most likely for Christine with all of them driving her wild and MC only just barely stopping short of fucking her. Sonya may get a foursome as well but in her case I think I could plausibly go all out, even including Christine knowing full well what everyone is doing with an up close and personal view of MC fucking her.
Also thinking about having Sonya catch MC fucking Jess and joining in while Christine is waiting in the Sauna. The vanilla game already hints at the possibility that they're being loud enough that someone hears them (at least if he spanks her). That leaves an option for a no holds barred Sonya/Jess/MC threesome.
I haven't decided how I'm going to handle branching yet for some of those scenes. I do want to preserve options for players who want to remain true to Christine so those players need to have a way to opt out of fucking both Jess and Sonya but I still want them to have options to get as close to the line as they want (or change their mind and cross it). I also want to have some stuff that is exclusive to each path so for example the foursome centered on Christine might be exclusive to players who remain true to her while players who don't get the Sonya foursome instead or something.
Some of the options in both the hot tub and the spa I also plan to have restrictions for depending on certain previous events. So for instance if you chose not to fuck Sonya on day 8 or day 11 and/or you haven't fucked Jess yet, then some of the Sonya/Jess options will be off the table. The lesbo options I will probably be more liberal with because hey, it's a porn game... So even if you're on the exclusive Christine path she'll probably still do the lesbo scenes unless MC stops it.