7-Eleven is offering all sorts of Detective Pikachu collaboration items, including Mr. Mime’s Mystery Flavor Slurpee. Just what is Mr. Mime’s Mystery Flavor Slurpee made of? That's up to the customers to figure out. With that said, 7-Eleven has supplied a statement to let everyone know that the one ingredient that's not in the drink is Mr. Mime himself.
The 7-Eleven promotion for POKÉMON Detective Pikachu is intended to be a fun incorporation of recognizable Pokémon from the upcoming movie. The mystery flavor element is intended to be thematically in line with the film which focuses on Detective Pikachu, a world class detective who has mysteries to solve. It is not intended to imply that the drinks are made of or by Mr. Mime and there is no lore associated with the drinks.