Thank you for all of your suggestions. I wanna share a bit of my current idea (which is not happening soon because I'm planning to do a simpler idea first).
Sorry to disappoint you
gedads but this idea I have in mind does involve MC's family members to some degree. What I have in mind is the combination of the regular NTR where a girlfriend/wife betrays the MC and family NTR where MC's family members such as mom, sister, aunt or all betray MC over someone else.
There's a Chinese web novel trope where MC gets in the way of the school bully who's rich so he decided to ruin MC's life by turning MC's girlfriend, mother and maybe sister too into his own sluts. And in the end MC could do nothing about it except accepting his fate as a pathetic cuck. I'm always baffled that I never read a story where the MC successfully fought back.
The premise of my idea is similar. The antagonist is going to be a rich jerk who MC gets into his way so he decided to ruin MC's life by fucking not only MC's girlfriend but his mother and sister too. (Maybe even MC's female friends too, depends) I plan to make the rival do something more sinister, like get MC's best friend into a car accident that MC can't prove the rival's behind it.
So the revenge part will start with his best friend hospitalized, his girlfriend stolen, his mother and sister didn’t believe him (not going to make them total cum sluts like those hentai comics who only see the rival as their man, but have enough trust in the rival that don't fully take MC's side), and MC decided to leave and plan his revenge.
Of course he's going to get some allies, someone who still believes him, or someone who got screwed over by the rival before. He's going to plan and train, then return, ready to let the rival taste his own medicine by taking every woman around him as well.
That's my plan so far, incest is possible in this that's why I think I probably cannot do this on patreon.