I hope nobody. The story is quite good and more realistic than the average porn game, and the characters are likeable; it would be a shame if this game turns into another mess where MC is an idiot or the female love interest an uncaring slut.
I hope the dev can keep up the positive vibe of the story and the personality of the characters, and also the healthy relationship between MC and his love. They are really cute together and it's unfortunately rare in porn games with NTR that a couple has a healthy and positive relationship. It's almost as if people think a dysfunctional relationship is required for NTR.
So, instead of MC doing stupid things and / or Ava turning into a spiteful slut, I hope there will be an antagonist who can attract and dominate Ava, exploit her weaknesses and just be somebody whom she can't say no to (and of course, this guy should not only get Ava, but also the mom, the sister and all the other women around MC

). Some kind of counter part to the cute, white knight that MC is; not a brute bully, but a smart, but selfish, uncaring and immoral boy.
MC shouldn't do anything wrong, he just shouldn't be this antagonist; the women love MC, but they submit (sexually) to the antagonist.
So in short, imho: NTR shouldn't happen because of a stupid, "tripping" MC or a vengeful girlfriend, but because of an interesting, "dangerous" antagonist.