Stumbled on this thread and decided to leave some thoughts.
Plot depends on what story You want to show, silly or more grounded. If We want to get away from tired cliches, We need to give mother believable motivation to fuck her own son. In real life it's happening in several cases, drunk mothers in troubled families, abusive mothers and control maniacs. Doubt You want to make some grim family drama, so let's drop unpleasant details and keep basic motivation.
So I see 2 story origins:
First, mother is insecure, conservative, controlling woman with chubby body, start mistakenly think that her son is homosexual after she saw how MC is watching some movie scene or UFC(as My sister once said it's most gayest thing She ever saw), her friend tells her that MC didn't see much of real women cause of her constantly spoon-feeding him and needs some urgent woman example, cause of her controlling nature she decides to be this example herself and starts to wear more and more feminine and explicit clothes in front of her oblivious son to save him from gayness.
Second, mother is bitchy wine mom in search of new husband, returns drunk from night party with her recent sugar daddy, old impotent rich geezer, and being drunk teases her son by pushing his face into her low cut cleavage. Rich geezer really likes it, and starts spend time at their house, slowly pushing her into more frisky "jokes" on her son, enticing her with money, jewelry and promises of marriage, telling her that it's all just an "innocent fun". It can be a branch where she either greedily keeps doing what geezer want or refuse and he starts to blackmail her with everything he filmed to do even more hardcore stuff.
Anyway, if we talk about MC, in all games he is just there, exists only to be horny and fuck his mom, function character. I would like to see MC slowly changing from a boy who had his first hard-on seeing his own mom, to somebody who starts to care and think about well-being of woman he fucks, thus becoming a man.
About things We like, one of incest fetish main elements is pregnancy risk, fucking a relative is already a terrible taboo, but getting pregnant after it is the worst, and that is why You need to push this theme, make mom allergic to latex or conservative pro-life no abortions activist, many deep creampies and scenes where they are interrupted and mom has to walk around and talk with cum dripping into her panties, well, I mean if You want to make best incest game You need to play highest note, and in case of incest it's a tons of rough risky vaginal fucking with messy creampies.
Second incest main element is risk of getting caught, and this means more scenes with people almost seeing them and hiding in plain sight, covering up creampie running down her thigh and stuffing her own panties into her pussy cause she didn't know where to hide them when somebody suddenly walked in.
I already wrote too much, whatever You do, do what YOU LIKE, it is Your game, stay safe and good luck.
P.S. Little details, please add little belly so she looked like a milf, now they look like perfect goddesses in their 30s, also breasts should sag slightly more too if they are natural, add some stretch marks to a top of their chests, and one thing that annoys Me, make their nipples darker with wide areolas, tired of games where milfs have small pink nipples of 20 years old girls, and the last thing, for a women who breastfed a babies their nipples should be located lower on their breasts, otherwise they look like ideal implants.