The main thing in an adult game is the argument, a strong base story is essential. It is what causes to want to play it, identify with the MC and, in some cases, enjoy some kind of hidden taboo fetish. When I play SexNote I don't find any of this and many plot bumps, children's dialogues and forced situations that don't help to plunge into history (which for now, it doesn't have). They are not exploiting the true potential they have in their hands. This is not a criticism, nor do I hope they have anything in mind. But with much more work and organization they could live up to DarkCookie.
1 - I do not understand what you wanted to do with the blood relationship between the characters, who is the real mother?, the dialogues do not match, the blonde is constantly telling the mc, perverted. That is not good, even being the maid. And the sister, whose daughter is she, is adopted?
This is something I brought up when you select the characters and how futanari or an artificial insemination method either changing the DNA to produce a male or using a relative from one of the females. It might turn you off. That is why it is optional or pending on something to mod and alter scenes if said individual really wants to include such a thing. The more you offer and the more you allow it to be a choice, the more people will play your game even if some of the choices they do not like.
An important detail for the story is that if there is incest, the characters resemble each other, they are familiar, right?
We have a brunette mother, the other blonde, the sister is redhead with freckles and the mc is brown and freckleless. "Genetic traits"
We also have magical powers and unrealistic giant dicks. Unless you are a biology major or someone who is absolutely awestruck by human accomplishments in science, most people really don't care. With genetic engineering, it can be explained very easily that the redhead has altered traits based on preferences from one mother. Very easy to surmise. Or if adopted, then that also can explain her physical features as well.
2 - The characters lack history, a motive, personality. Who is who and why they do what they do.
This is true.
Base story suggestion: MC - When my parents divorced, my father, a great lawyer, kept all material possessions. Leaving my mother with nothing, no money, no clothes and no house. This left her shattered. (That would explain why she is naked under her robe, in front of her son, in the kitchen with a glass of wine in the morning).
I could have gone with my father but I couldn't leave her like that, that's why we are now living in my aunt's house, a strong, feminist and independent woman who never needs anything from anyone. (Needless to say he doesn't love me, he thinks I look a lot like my father)
This is possible if the option was given. Remember there are options in the roles between individuals and this backstory wouldn't fit if the player choose to make the mother a maid and the aunt a boss, right?
3 - We have a book with sexual powers... (This is the real mc, not the boy. Without a book there is no story) and first of all the sex scenes have to have some common thread related to the book, not just fornicating. Where is the corruption and progress in the game? What is the use of agamoto eye if women are already half-naked? Another variable to this spell would be to be able to see through the walls.
The MC does not know how to masturbate, what kind of teenager in the 21st century does not know? So... thanks to some kind of mind control or suggestion spell, the mc should get his mother's favor. (Do not take lightly, of all the characters the mother has to be the most difficult to corrupt)
Three nights watching videos and we already get oral sex, without magic? it does not work like that.
Very true.
4 - A giant cock, really? This should go later in the game.
True. More that is an option from the beginning of the story. Cock progression, yes or no. If not, then big cock or little cock throughout the story or only relevant with the story.
5 - The mother and stepmother (or whatever) should be heterosexual and the MC with magic can be corrupting them, lesbian scenes should be progressive.
No. This I disagree with. You are turning homosexuality or bisexuality into a fetish. I know it is, but at the same time heterosexuality can be a fetish if you are talking to people who are sexually different from yourself. More likely they are more open with their sexuality rather than changing from one type to the other.
6 - In the dialogues the previous point should be reflected. Type, - What are we doing? - or - This is wrong - or - We shouldn't - etc.
Depending on choices made earlier. In fact, those choices could had been made at the beginning when the book is made and written in the book. Maybe the boy thinks of it as a diary and uses it as such. Thereby, whenever he inputs something into the book, it becomes true. However, it is up to the boy to make those decisions and he becomes bolder as the game progresses with of course play input pending those decision. Therefore, it is the book and the boy rather than just the book that is the key element behind the story.
7 - Mind control, suggestion, extra body trips. They are some of the spells that the MC could use at the beginning of the game.
Eg: through extra body trips the MC could get into dreams and get to see women's fantasies or worst fears (this opens the possibility of two paths of love or domination). With the suggestion, manifest them, let the women see them (hallucinations).
And with mind control, turn women into puppets of their perverted will.
Or just altering reality as the boy becomes bolder and explore more possibilities. Make it relevant to how the boy may progress through each scenario in realistic terms of how a normal person may proceed with choices from the player to alter those changes based on personal preference, thereby not making people see futanari, vore, spontaneous pregnancy, or public nudity in a game when it is not their thing.
8 - Mini games, side characters, side missions, all that is important, but without a good base story it collapses like a house of cards.
Very true.
...or a few days for the official public release 
