The British stars are developing a film about two lovers who "ditch their partners to be together," according to Britain's Daily Mail newspaper.
Director Dominic Savage told the publication that he has met with both James and Cooper for discussions about the flick, which has tentatively been titled "Is This Love?"
"It's about the kind of exciting, absolutely-has-to-happen love," the filmmaker commented.
No further details, such as other potential cast members or a release date, were shared.
However, Savage indicated that he was looking to keep the cast small and potentially begin filming in July 2019. The trio may finance the film themselves.
"The other characters are referred to. Alluded to. You might hear them on the other end of a phone," he said, adding: "The idea is that if we keep the budget really small, you can just about make it."
The Hollywood actors both appeared in "Mamma Mia! Here We Go Again", with James portraying a younger version of Meryl Streep's character Donna from the original 2008 musical movie, and Cooper reprising his role as Sky.
Meanwhile, Cooper recently worked with Savage on drama "The Escape", in which he starred opposite Gemma Arterton playing a woman who makes an extraordinary decision which will change her life forever.
While James has a full slate of films on the go, as it was announced earlier this week that she is to star opposite Armie Hammer in a new adaptation of Daphne du Maurier's "Rebecca". The 29-year-old will next appear in an untitled film being made by Danny Boyle and Richard Curtis.