The Avengers films did exactly that. They filmed characters with random costumes because they hadn't done the designs yet. Then replaced the costumes with CG later.
Some of the reason they gave for that at the time wasn't just saving on real world costume designers by outsourcing it to "cheap" VFX teams, but also for added "security" to avoid "spoilers" leaking to the press in tabloid photos.
(Spoilers of a sort: My favorite meta-joke on all this was Spider-Man: Far From Home's "primary" Mysterio costume which had tabloids and fans doing all sorts of speculation what the CG composited costume on top of it would look like, given the Avengers situation, only to find out that was actually the outfit for many of Mysterio's scenes because it very well fit the character in that medium.)
You could just assemble a cast, bring them into a greenscreen studio, and shoot a complete matrix of poses and angles and lighting variations. Then synthesize the film from that when someone decides what it should be about.