Audiences have already been introduced to a few different DC superheroines, so it makes sense to develop films around characters that already exist in the DC Extended Universe. Katana would be one of the most interesting choices because of her backstory.
First introduced in Suicide Squad, Katana left a memorable impression in the limited time that she was on screen. Stoic, formidable and strong, Tatsu Yamashiro wields a mystical katana called Soultaker that contains the spirit of her husband. She’s determined to avenge his death and the dishonour that was brought to her family, which could form the backdrop of a solo film.
DC have the option to go with an authentic Japanese story by featuring the film in Japan and making sure it’s presented in Japanese. Katana could take on a criminal organisation and track down leads about her husband’s death. The movie could even take inspiration from the Japanese horror genre by including supernatural creatures that Katana needs to defeat, allowing the soul-sword to shine through.