I would make six different versions of the movie, three with a male shepard, three with a female shepard, one each pure renegade, one each pure paragon, and one each be more than a little ambiguous. This wouldn't be as hard to do as you'd think; under normal circumstances, your editors would only use the best takes from each scene to put the movie together in the editing room. In this case, you film the exact same scene at the exact same time with the male AND female lead and have the editors take the best takes from each version of the scene. Paragon Shepards would spare the Rachni, sacrifice Ashley, sleep with Kaiden (Liara for the male), argue Saren into killing himself, and rescue the Council. Renegade Shepards would kill the Rachni, sacrifice Kaiden, sleep with Ashley (or Liara for the female), talk Saren into lowering his guard so Garrus could snipe him to death, and tell the council "Fuck you, shoulda listened to me when I warned you" as the Geth board the Destiny Ascension and rip them to shreds. Middle-ground Shepards would spare the rachni, sacrifice whichever squad member they were more likely to sleep with (and therefore get no play at all in this movie), argue Saren into killing himself, and then tell the fleet to focus on Sovereign while sending one last message to the council saying "Forgive me, councilors, but the whole galaxy is at stake here!"
Then I would do everything possible to make sure that you couldn't actually know which version of the movie you were going to see when you went to the theaters, and make sure that absolutely nobody has any idea that the film is actually put together in multiple versions.
Then I would gleefully watch critics and moviegoers totally loose their shit as they begin to realize what's going on.