It's possible to present the audience with a character who seems like a stereotype at first, and then introduce complications as the story goes on. ( Patient Childhood Love Interest, Meido, Ojou / Sheltered Aristocrat, Not Blood Siblings, Troubled, but Cute, Token Mini-Moe.) Add some unique-but-not-bland characters, or at least add some variety. Stereotypes: some character lists in bishoujo and otome games can seem like checklists of fan-favorite character types. Just show the audience that she has plausible strengths and real weaknesses. Mary Sue: if your main character is female, some fans may be quick to label her a Mary Sue, or as some sort of stand-in for Fangirls. However, there is reason why Dating Sims use this character less and less. Note that Featureless Protagonist is exception to this. You should ask questions such as, "Why would anyone like this person?" Remember, it could get worse - if your story involves horror/dark themes, and if your main character is doing horrible things, then your audience may want to see themselves as anyone but that character. So do your best to find an alternative to "boring" and "obnoxious" characters. Remember, you want your audience to sympathize with the main character, or perhaps imagine they could be him. Also, a main male character can sometimes be a Marty Stu who gets all the girls with no effort.
Just like in the Write a Magical Girlfriend Series entry, a male main character may be an Unlucky Everydude. Or you can create a game filled with challenges and/or puzzles, and make it difficult to get to a sex scene. You can create a love story in which a single sex scene is an important landmark for the characters. You can include hardcore scenes at important moments. If you have decided to create an adult game, then you need to decide how sex will be incorporated into the story or the gameplay. Don't let all of them be happy, because if the main character is not allowed to fail, then your story/game may lose its emotional impact. These can often vary from good to bad to nihilistic. Nearly all dating sims have Multiple Endings. However, there are examples of ren'ai games which take place in diverse genres, including but not limited to fantasy, science fiction, horror. If your setting is realistic, then make sure you can portray it in a way that the audience will find interesting.
Likewise, many commercial games have a Slice of Life setting. Note that harems do not necessarily exist in doujin visual novels. If your game will have one, then find a way to make it logical. Most commercial ren'ai games feature a harem. Therefore, you need to devise interesting ways for their feelings to develop. Unless you are making a story which is the equivalent of "gen" Fan Fic, you will have a main character who falls in love with one or more people, or vice versa. You could also make them bisexual if you want love interests of both genders, but don't put too many options or it can be overwhelming. Any of them could have one or more transgender people or bifauxnen as potential love interests. Any of the above could conceivably be crossdressers.