One simple thing I'd like to see, and maybe someone might've already mentioned(sorry I didn't read other comments) is characters that act their age. I'm tired of seeing an adult male getting embarrassed because he hugged his sister and is like "awww oh my god I have a boner!! What do I do now?" and the story goes on as if that silly nonsense makes sense. I don't think that even when you're 10 years old you go around having random erections for hugging people, specially your family members. At least I didn't. Either that or when they act like a horny teenager.
I also find it very silly the typical "unaware MILF". People make these bombshell hot milfs and when there's some sort of corruption or seduction from the MC towards his mom, they're usually portrayed like "uhh what's this I'm feeling". Bitch, you've fucked multiple times and has multiple children, you know what lust, sex and sexual desire is.
Entertainment is important in AVN's and, of course, the sexual situations. Obviously you won't make a "realistic" approach to incest because, realistically, it's very difficult and, in most cases, impossible to seduce your mother/sister. But we already use our suspension of disbelief to enjoy the novels, I don't think there's a need to dumb down the characters to make things happen between them.
I think that if novels focused more in the "guilty" factor of incest (it's taboo, it's looked down upon, it's a bit weird, etc) instead of dumbing down/hornifying the characters to make them more open to this type of relationship, things could be more organic.
Story wise, I think that someting that could be different from the rest is finally making a character that has graduated university and is maybe starting a job or is already stablished in his position, maybe finding his place in something, maybe turning his hobby and passion into his main focus or anything of that kind. How many "college atendees" do we have as protagonists of these novels? Of course, I say this if the novel is supposed to take place in the modern world, but if it's a fantasy/sci-fi that doesn't have these aspects then it's another story.