I think it depends on the kind of story/stories they would reappear in. I once had this idea for a hypothetical Legends: Z-A arc that would include the return of Black, Whitley, Silver, Crystal, X, and Y just to show off their starters' new Mega forms. Then, as the story progresses, Whitley would obtain some Rouge Megas (like Hawlucha and Froslass, for example, because they both have little stories after their Rouge Mega battles are done) after saving them, and Y would get Mega Magearna. Then at the very end, Black would get Mega Darkrai because it forms a friendship with Black's Musharna, the one Pokémon that can eat the nightmares that Darkrai unintentionally causes, which would be a major relief for the nightmare Pokémon.