One characteristic is the crossover of different power systems. In these fanfics, the quirks from BnHA and the psychic powers from Mob Psycho 100 often interact in unique ways.
Hard to say. Maybe it will if there's enough demand and the story has more to tell. But sometimes, even popular mangas come to an end. We'll just have to wait and see.
Mob Psycho manga has indeed finished. It wrapped up with a satisfying conclusion for fans. The author completed the narrative and tied up all the loose ends.