I saw someone have an idea for Victini but I wanna give one I had based on one of the weird facts about Victini. I think Victini should jump the player on the way to the 5th chapter, still applying the mythical rule where if you don't capture her, you can't catch her again.
Victini for some reason is the first Pokémon of gen 5 pokedex wise which is a break from the normal format of having legends and mythical be the last Pokémon pokedex wise in a new generation so I think having her basically jump the theoretical line by attacking in the intro would be a smart reference to that.
(Edit: I do realize this can theoretically cut out liberty garden so perhaps make this not a capture event but just her appearing, she was in a lighthouse)
(Edit 2: Oh even better, you can make a battle you can't win, hint about Victinis boundless energy like the other user talked about, setting up for Victinis encounter proper at the liberty garden)