Yep, the avn writing for males is pretty bad...it's like another post here said regarding male friend characters in avns - can be similarly applied to mcs; often we never get a deeper sense of who they are(despite lengthy monologues). What do they pride themselves on? What are they afraid of? 9/10 times it's either captain bland or Chaz the super-fucker.
I think a character like Rhett Butler is believable and interesting, even if some people would claim he's Mitchell's fantasy chad
Actually, the entire Gone With The Wind story would make a great avn, full of sharing, cheating, spiteful and revenge-fucking, while still not feeling horribly depressing if the main character were as cynical and self-confident as Mr. Butler. Scarlett, as she's written, is not at all a lovable FLI, more of a pursued trophy, one of Rhett's favoured lost causes. He sees her try to steal Ashley away from Melanie, which in an adult story could include her succeeding in making him cheat; Mellie sees no harm in it, takes it in stride, it's just the way her beloved cousin Scarlett is, so full of life

She becomes Rhett (the MC)'s close friend and confidante, and while he witnesses all this, sometimes very closely, not jacking off like a fucking loser, but calmly watching, savouring his "lost cause" with an unfettering belief that he'll win her over in the end, he and Mellie have conversations about it and Rhett tries to corrupt Mellie, which he can succeed in.
Make "AVN Mellie" pursue a little of the liveliness of Scarlett, maybe have Rhett wingman her to other guys - while Rhett himself can fuck almost any woman, lady or prostitute, that he likes, but won't do it with Melanie, preserving a distinct relationship with her, rather than going full harem. Perhaps there's an opportunity to make this a choice, whether Rhett should help Mellie loosen up and enjoy being pleasured, "corrupting" her, but without fucking her; or taking her for himself; or not corrupting her at all.
Meanwhile, he sees Scarlett marry George out of a spiteful impulse, while later still cucking him with Ashley. Rhett sees Scarlett progress form an impulsive and selfish girl to a no less impulsive and selfish woman, but more capable of fighting for her goals - she's ripe for him, and he makes his move. When George dies, he offers her a rationale for marrying him, and she says yes, mainly to save herself from the boredom of widowhood. Add their first fuck where the original has their scandalous waltz. Honey moon trip, then back to reality, Rhett still believes she's in love with Ashley - in fact, he may witness her cheating with him several times, but doesn't care. That's just the Scarlett he fancies, a corrupt, selfish "princess" he can parade with and now fuck at times, regardless of her escapades. He's not the helpless, betrayed husband, not the pervy cuckold who jacks off to her being fucked by his "betters", but a confident, unscrupulous man who enjoys fucking with socially accepted behaviour and roles. He marries Scarlett knowing fully that she'll cheat and he doesn't care. He can cheat back on her, but not because of revenge - but because they're alike, just like he tells Scarlett (when, where and with whom he'll cheat is an opportunity for more flavour choices). He probably enjoys having his prostitute confidantes, with whom he gossips about the hypocrisy of the well-to-do society - especially one Belle Watling, who's an important side character. There's genuine affection between them.
Rhett's emotional tethers in this story are with Belle and Melanie, one which he corrupts, but not entirely (makes for good character development if Mellie loosens up a good bit, while remaining essentially a loving character who "sees no evil"), another who's just as disillusioned about people and society as he is. This is already three "love interests", none of them has to be an exclusive route, there's no harem either, each have their own role.
Throughout the story, Ashley is not at all an antagonist. He's reluctant, pursued with guilt, clinging to social norm albeit admitting his irresistible attraction to Scarlett, and she's the one playing him. This might please players who prefer to have the FLI having sex with a steady friend, rather than randos and as long as there's no "scoring" for the guy or humiliation of the MC, I'm game. Ashley is a romantic idealist, who can't live up to his highest standards, which is both his flaw and his redeeming quality - he accepts he's no better than Rhett, to whom he may later try to confess his affair and his love of Scarlett, and there can be mutual respect between them. Maybe take it even further and create an arc where Rhett and Ashley can get drunk and dabble in a bisexual adventure, another optional blow on stereotypes - and the opportunity to add another tag to the game LOL Perhaps make it even more perverse and have it be Scarlett's fantasy, with optional ways it could play out...
Not sure about the tragic part of the story, whether or not it fits an avn, but maybe it does. Mellie dies of child birth (Rhett knowing her son could well not be Ashley's), he's devastated and starts being less admiring of Scarlett's selfishness, believing that she now has her way clear to make her relationship with Ashley less secretive, free to achieve her lifetime goal in spite of him, Rhett. His first falter in confidence, part of a dark chapter that can be resolved to different endings. I'm not a fan of children dying in stories, especially stories that focus around eroticism, so - skip the death of his and Scarlett's daughter, let Mellie's death be the trigger of the dark portion of the story. There could be flavour choices throughout the game, but now there's a chance to make for branching endings. Does he forgive Scarlett and accept her confession of love? Does he demand that she forget Ashley and become a faithful wife, or accept her continued indulgence, with Ashley as well as with others, but now reassured of her true love and admiration, perhaps having Scarlett as a hotwife? Does he angrily dismiss her, but stays at home, pushing her back to Ashley and to a bitter cuckold/NTR ending? Does he leave, as in the original, joining Belle on a trip to Europe? Does he set fire to the house, killing Scarlett and Ashley, ending in total tragedy? I know which I'd be more likely to choose, but all endings could have a few scenes directed at the players' preference for good or bad endings, vindication or defeat, violent or passive jealousy, etc...