Don't fall into these common mistakes:
Everyone enjoys fantasy but keep the story and characters somewhat realistic. Don't ruin the game with an unrealistic fantasy. The most common one is "I have a huge dick and everyone worships it and throws themselves at me for no other reason."
Don't turn all females in the game into sluts. Make it a challenge to score with the main characters in the game! But also, don't tease for so long that the player loses interest.
Keep everyone's body proportions realistic. Don't give females all H cup breasts and guys horse dicks.
Give the NPCs in the game realistic personalities and varied personalities.
The story and characters are the key to a game's success! Make the story and characters meaningful!
If you give the player choices, make most of them matter. In far too many visual novels, nearly all choices follow the same path. There is only the illusion of choice.
As already mentioned, set realistic goals and expectations. Break the game up into small pieces, days, or chapters.
There needs to be some sort of struggle that the players has to overcome in the game. It can't all be rainbows and butterflies (looking at you WVM).
There needs to be consequences for some choices. The harder the choice the better. The choice needs to be more involved other than the common, "who do I want to fuck today."
As already mentioned, if you aren't good with English or aren't a native English speaker, find a proofreader to fix all the mistakes you make. I can't tell you how many games have been tainted by poor English. If you get a proofreader and use one from the start, it will be much easier to maintain. Once you are half way through the game it is very difficult to go back and correct all the bad English.
Good luck!
Just, thanks for the tips !
Clear and accurate.