With Ash must come Brock, his longtime companion and pseudo-mentor. The Pokémon breeder is almost as charismatic as Ash, and acts as a peacekeeper and caretaker. Dev Patel could certainly match the maturity of Brock and make for a great compliment to Gleeson's Ash.
There is one person who can understand Detective Pikachu: Tim Goodman. Gamers play as the inconspicuous character, assisting the private eye in figuring out mysteries. It's a difficult role—Goodman must function as an avatar for the audience, while being an energetic screen presence in his own right— but we know Tony Revolori could accomplish the tall task.
With Brock and Ash must also come Misty, the final piece of the franchise's main trio. While at times nurturing, she's known for her ability to blow up. Hailee Steinfeld is brimming with talent, and has already proved her ability to balance calm and fiery perseverance in the Coen Brothers' True Grit (2010).
Detective Pikachu lacks the physical capabilities of his special brethren yet makes up for it with great acumen and investigative skills. While the detective is able to talk, no one seems understand him, and he is consequently disgruntled and gruff (he is a P.I. after all). While American gamers petitioned to have the illustrious Danny DeVito voice the Pokémon, the actor declined, which is fine as he would have taken away from the visceral experience of a talking Pikachu. Instead, we offer Karl Urban, whose comically hoarse and hammy performance as Bones in the Star Trek franchise reboot would seamlessly translate to a detective role.
Any Pokémon movie, regardless of source material, would be lost without Ash Ketchum. The franchises's longtime hero will no doubt be featured in the film, an always likeable, go-getting, moral character. We'd like to see Game of Thrones's Jack Gleeson translate Joffrey Baratheon's malicious energy into something more noble.
Squirtle is almost as iconic as Pikachu, and we'd be heartbroken if we didn't see the blue, water-spurting Pokémon in the live action film. Vin Diesel has proven himself to be incredibly adept at saying a few words with great heart (see: Guardians of the Galaxy's Groot), and Squirtle may just be the role he was born to play.