A young boy from the "real" world dies and ends up wandering as a soul in the world of MHA until he eventually enters the body of a woman who is announcing to her husband that she's pregnant. (This can be changed.)
Either he is born before the main cast, in the generation of Hawks, as Danzo Hagakure, the older brother of Toru Hagakure, and later joins the Meta Liberation Army or the League of Villains.
Or he becomes another fetus, and on the day of birth he is named Danzo Hagakure, alongside another baby who will be invisible.
Danzo Hagakure, the twin or older brother of Toru Hagakure, will have light green hair and light green eyes just like she would have if she weren't invisible. However, when he enters his ghost mode, his hair turns white and his eyes become fluorescent green.
Because Dan becomes Toru's twin or older brother, her powers will be greatly enhanced, more specifically, they will become close to those of Invisible Woman (from Fantastic Four).
Dan, of course, will have the powers of Danny Fenton:
Human/ghost physiology, transformation, superhuman strength, superhuman durability, superhuman speed, superhuman reflexes, superhuman agility, superhuman balance, superhuman stamina, healing factor, invisibility, intangibility, flight, possession, energy manipulation, pyrokinesis, telekinesis, X-ray vision, teleportation (possible), cryokinesis (later in the story), Ghostly Wail (later in the series), combat skills, high intelligence, tactical skills, enhanced learning, leadership, and multilingualism.
Since it is a Quirk, it has a drawback: when he activates it, his personality changes and he becomes more violent. (He may potentially become a villain.)
The first chapters will focus on introducing family and friendships, Quirk training, and Danzo's personality / dual personality.
After that, the story can move on to the U.A. entrance exams, the usual early arcs that many people enjoy: class orientation, the first villain attack, and then the tournament if Danzo is written as Toru's twin.
Now the next step is to find a clear storyline and goals for the character, romance, etc...