I tend not to like a lot of harem anime yet love others. It took me a while to realize why but its because of the plot. So my advice to you is don’t go into this solely thinking of it as a harem anime.
The more successful harem/reverse harem anime have some kind of plot or quirk going for it. When writing/coding your game, think to yourself, “If I took the romance out of this would it still be good/at least coherent?”
Another thing is, treat your characters like realistic people. They should have lives outside of the main character’s direct line of sight/influence unless it’s part of the character’s motivation or something (i.e. a yandere character might specifically live for the MC to an unhealthy level). This applies to the MC as well as the love interests.
I’m not trying to tell you what to do, just giving advice because I’ve seen so many bad harem anime that could have been decent if they just followed those 2 of the many rules of good storytelling.