Hellsing ultimate inspired fic xover
All for one gave his sick quirkless brother a stock piling quirk. He found out later his brother did have a quirk. A lot more similar to his than anyone realized. The stockpiling quirk he gave his brother helped, but his brother had been sick because his dietary needs were not being met. His quirk was not something that was passed on it was an infection. This version is what if the first holder of one for alls quirk was not passing the quirk on. It was more akin to a vampires curse the previous holders are not vestiges they are the previous holders at least their souls.
Izuku when he received the quirk, one for all awoke his latent quirk. It was very minor shadow manipulation, in combination with one for all the power truly matured it didn't break his body it changed it. Making him more like alucard from Helsing just not as violent problem is the previous holders can use those shadows to manifest and are making his life a bit difficult.