Something I'd like to see more games focus on is jealousy. I've been thinking it would be fun to do a game that's sort of a re-imagined version of Indecent Proposal. In my case I would want to reverse the roles but it would work either way. So we have co-main characters (FC and MC) who are engaged and in serious financial trouble and a mysterious rich woman makes them an offer to solve their financial issues if FC agrees to send MC to her every night for a week. It's implied that this means she wants to fuck him but that's not actually explicitly required, just hinted at.
The basic idea would be that you play through the first time as the female main character (FC) and this playthrough would focus on the backstory leading up to the proposal and how much FC and MC love each other, FC's mental state as her mind plays through various nightmare scenarios of what might be happening at this womans house, and then their interactions when she drops him off again in the morning. A further condition of this proposal would be that they can't talk about what happened and that she will have surveillance equipment installed in their house for the entire week to enforce this.
As you might guess, this woman's real motive is that she's into mind games. She might actually want to fuck MC, but her real target is to get in their heads. So to spice up the original proposal, she makes additional offers to both of them individually each day which affects what happens each night. These offers are set up as a tug of war between trust and jealousy.
As an example let's say the first night, this woman sends a limo to pick MC up and takes FC aside for a little talk before heading out. She asks FC how much she trusts MC to restrain himself, then shows her a live video feed of a couple of incredibly hot girls in the limo with him right then already starting in on lap dances with him. She offers a choice. She can just roll the dice and wonder what happened that night or she can give the ok for the girls in the limo to unzip his pants and suck him off right then and there while she watches with the promise that anything else that happens that night won't go any farther than that.
She plays similar games with MC when you get to that playthrough. Using the first day as an example for instance, she shows MC surveillance footage of FC watching MC in the limo (whether she took the deal or not at the very least he knows that she saw lapdancing/etc with these other girls). She doesn't show him any footage about her deal though. What she does tell him though is that she plans to have her driver deliver a video tape showing her everything that happened that night in exchange for a "favor". The catch is, he doesn't get to know what that favor is and as far as he knows, she has no clue what will be on that video. She offers him a choice. He can roll the dice and trust that she won't take this "favor", or he can agree to some other condition she wants and in exchange this driver will not offer her the deal.
So depending on the player choices in each play through there could be wildly different outcomes depending on how trusting each character was, how jealous they were, and how much of a voyeur they were.