A guy has a harem of girls who are in love with him, but he's indecisive and dence so he takes too long to make a move on any of them, and then all the girls just leave his company one by one to go and live a fulfilling life. In the end, the protagonist is alone, and makes a bunch of excuses for why he could never be in a relationship, and then moves on with his life to focus on career or something.
More of the reverse, as usually the number of female characters around them increases as the story goes on. In this case it's a start high end low function instead.
I like the oneshot i read some way back about the dude who is pretty much a harem MC with all the tropes cast, little sister, big sister, childhood friend and what not, but he's gay and like his ikemen classmate.
I had also read that one and I was wishing for the story to be continue and there was this idea from a novel I've read, where the protag wants to create a harem and hook up with different girls, however, all the girls find out and break up with him and give him ED.
I read a similar plot to that, which was in a bigger/overarching story. A man wanted to create a harem, and he secretly collected girlfriends for years. Eventually he gets found out and all those girls planned revenge. However, one of the girls contracted a sexual disease from him and decided to murder him for revenge as she knew she was going to die and had nothing to lose compared to the other girls And did we read the same novel?
I get what you're going at, but for a series that's kinda boring. One of my gripes with the harem genre is the wasted romance potential. Take Nisekoi for example. I have a love and hate relationship with it. I enjoyed it and at least appreciated how it ended with the MC settling on one(the expected route) girl and the MC dealing with changing love over the series, yet, it's a harem with frustrating elements and I'm the type that want everyone to be happy despite knowing it's not realistic. The heroines of Nisekoi each individually have a great romance story potential due to their character, background, etc. For me, the appeal of a harem isn't really the having lots of girls part. It's the different personalities, interactions, background, etc. along with the many different first meetings, relationship direction, and confessions. Each heroine would have been a great non-harem romance story/series. That's why I find Quick Transmitigation, Puedo-Harem, and the Routes aspect of VN appealing and brilliant. I also found Golden Times appealing due to its childhood friend/love interest character moving on and finding her own love aspect so good. My idea for a story would not focus on the MC and his loss of the harem, since f*ck him. Maybe only at the start or in snippets for that schadenfreude. I'd focus on the heroines moving on, finding fullfilling love/romance and healthy relationships, and making their own happy endings. Another idea, although I personally would struggle with liking or hating it due to the angst, is a sort of twist on the scenario with a selfless MC who can see the future or see many possible futures and tries to set these girls up with the partner that make them the most happy. He gets to know them, starts to care for them, but always sees that they' be better with someone else. I feel the MC would have to go through a lot of pain especially in cases like acting like an villain in some scenarios or forcefully breaking off relationships/bridges for the girls. He'd constantly feel inadequate or undeserving of their love and have to struggle with wanting their happiness more the more he cares for them. A vicious cycle. Maybe in the end he'd learn that love isn't really about finding the perfect person or that someone be happier with someone else. It's about choice. It's about willing to give up that potential happiness because that's not what love is about. It's about sacrifice, about the present, about both the good and bad, etc. The "But I want you. I choice you. It doesn't matter if I'd somehow be happier with that guy in a possible future. You are the person I love. Not him. Not anyone else. I love you." direction.
I remember writing up a draft for a story similar to your idea there when I was in a brooding mood awhile ago. It was about a guy who got drunk and cheated on his wife killing himself, and finding himself suddenly possessing the body of a high schooler. He remains in an constant state of regret, and always focuses on how he doesn't deserve anything good to happen to him, so he just works from the side lines setting up everyone around him to be happy and find love and all that jazz. In the end, I couldn't write out the whole thing since I actually have no clue how romance works, so I gave up.
I also can't write realistic romances either since I've never been in a relationship nor seen good ones irl. However, with the amount of romance novels/mangas/animes I've read and watched especially of the josei and shoujo genre, I could probably write a passable enough idealistic/rose-tinted/fantastical one.