I had a manga idea when I was young. The concept was a young man who is pressured to do something with his life after high school accidentally wanders into a fight between two superhumans who turn out to be vampires. He recognizes one of them as a childhood friend of his from school. He helps her get away but she was desperate for blood from her injuries and sucks him off. She meant to only take a little but she does it way too hard and the MC goes unconscious. He wakes up in a secret underground vampire society having been turned into one and the girl's underling. After that I think it basically kind of turns into a combination of FMA and YYH with the MC and the girl going on missions to stop other bad vampires and making a team who don't get along with each other at first but eventually become friends. Also the MC's buddy from school would turn out to be a werewolf who are supposed to be enemies and one of his classmates would be part of a group of vampire hunters
Each vampire would have different weapons. The MC would use one of those arm blades, the girl a katana, another girl on the team who is like a noble Vampire, uses daggers, her butler a staff, another guy a giant Buster sword and werewolf buddy would use martial arts. Vampires would have these bats that temporarily sucked enough of their vampire blood and replaced with human cells which would last for a day so they could do human shit if they needed.
Also, I was reading Negima at the time so I loved the idea of having a tournament and having the big final boss be this fucking crazy strong guy whow as a bro so I made him a combination of Jack Rakan and Toguro.
Have you ever read the Young Adult series Cirque du Freak? It was largely based off of that concept but like as a shounen.