A production list says that Detective Pikachu's film production will start on January 15th. Refinement will take place in London. But at this time, there is still no other information about the film.
Detective Pikachu is one version of Pikachu. Unlike Ash's Pikachu that appears in the anime, Detective Pikachu is not interested in fighting with other Pokémon and coaches. It has a purpose to solve the mysteries that exist.
The Detective Pikachu character itself appears in the Nintendo 3DS game released in Japan. Then, Detective Pikachu has its own game, though not big. From the animated film, Ash's Pikachu is quite friendly and can interact with humans.