I want to help you with dialogues in H-scenes, but my English is still far from perfect as I'm not native. So I can make some advices rather than make dialogues myself.
I'm not a native speaker either.
First of all you need to create a contrast between sex with MC and sex with NTR guy. Best way for it is to make sex with MC very vanilla and/or sex where she takes active dominant position. And sex with NTR guy should be hardcore and even humiliating, where she slowly (or not so slowly) become very submissive and lustful. In end stage she should ask permission of NTR guy, beg him to fuck her (and he will tease her and make her make efforts to deserve the fuck), call him master (or similar way to make her low position clear), saying how much she loves his cock. BUT - this is end stage, hard thing is to make her fall not too soon, in a natural way.
Most basic scenes are already drafted and only need a look-over before I finalize them. The scenes after that are planned out but can still be added to/changed heavily.
I went with the sex with the protagonist being vanilla for contrast, but also tend to skip over/shorten sex scenes between the protagonist and his wife since they're not the interesting part (not to me, at least). Asking for permission, calling him master, and saying how she loves his cock are all in there already, although it's still somewhat forced (as in saying it because NTR guy wants her to, not because she wants to) until about the 75% story completion mark.
Here example of such a game.
https://vndb.org/v16881
But most of the best NTR games basically follow such direction of dialogues. I wish it was somehow helpful.
Haven't played that one (yet), but one of Jinsei's games' CG set is probably what got me into the NTR/Moral Degeneration with appearance changes combo. Unfortunately having to unlock the female perspective scenes in games like these is a downer for me personally (which is why I chose to let the player chose whether he wants to see them the first time around or not in my project).
And yes, thank you for the help.
Great looking stuff! Are you making the cultural background Japanese setting or USA setting?
Neutral. I'm not from the USA (or Japan), but would never use my own country as the cultural background. The country won't be mentioned, but I use the same premium copyright free backgrounds many Japanese eroge use and those are based on Japanese buildings/parks/etc. I also recently wrote out a scene where someone's shoes being left at the front door is being considered normal, which you can also consider a Japanese thing (although I personally do it as well in real life). But I don't go out of my way to conform to Japanese social or cultural etiquette so there's probably some western behavior as well. Personally I see it as a modern, western, non-existing country, but I don't think it's too hard to self-insert your own country into it if you like that.
1. Wedding ring play - ring removed from her finger and thrown to her tummy, and then the lover cums all over her tummy tainting the ring and then make her wear back on her finger
There's something with Fiona's wedding ring in the story already, but not quite the above. Won't spoil it for now. In the top image from my original post (which is the main menu background), if you look well or enlarge it, you can also see a broken wedding ring lying in between the two women.
2. Cosplay - slutty school girl, naked apron, etc
Right now I've planned for a scene with a slutty wedding dress outfit and slutty "pig" outfit (like the one in Kedamono). Now that you mention it, I'll consider a too-tight school girl outfit as well.
3. Condom play -making the wife remove the condom.
There's a scene early on in the story that's somewhat the opposite, but this is a good idea as well. I'll probably incorporate it later in the story in a way.
4. Wedding - the wife will dress as a bride and marry the lover, without divorcing her husband. The wife will not divorce her husband because the lover will not divorce his wife. After that point, the wife will refer the lover as the only true husband of hers. The lover will still treat the wife only as sex toy.
There's a somewhat similar wedding scene planned with the above wedding dress cosplay and wedding ring play.
5. Pregnancy - NTR is not complete until the wife gets pregnant lol...
There's at least one ending planned focusing on pregnancy. I'm still thinking about her being pregnant but it not being the focal point in another ending.
It's just a few of my suggestions...
Thank you, I appreciate them.
Shine