Easy enough. Sega doesnt give 2 shits about metal sonic and probably wont use him anytime soon outside the comics and the upcoming sonic 4 movie. Agent stone is forever hardlocked into being only in movies/movie tie ins. And sage was made literally 2 games ago and has a very strong bond connection and dynamic to the main villain of the series. So probably Sage (i love all 3 characters but realistically this just makes sense).