By the end of Sonic the Hedgehog 3, the Sonic cinematic universe has gotten considerably bigger. Shadow the Hedgehog and Gerald Robotnik expand the Sonic family, and a mid-credits scene promises to add even more fan-favorite characters for the eventual sequel, Sonic the Hedgehog 4. As Sonic 3 hits digital release this week, Polygon took time to speak to Sonic the Hedgehog franchise director Jeff Fowler about the future of the series and what it holds for two new characters.In Sonic 3s mid-credits stinger, an army of Metal Sonics show up to neutralize Sonic. But a savior arrives: Amy Rose, cloaked and armed with a giant mallet, and she lays waste to the robotic Sonic clones. Neither Metal Sonic nor Amy speaks during the scene, which leaves their places in the story open-ended for now.Read on for Fowlers insight into the addition of Amy Rose and Metal Sonic, who have been teed up to appear in a fourth live-action Sonic the Hedgehog movie.Polygon: When did you know you were going to expand the franchise with Metal Sonic and Amy Rose?Jeff Fowler: We actually had the idea pretty early, during storyboarding. We love doing these teasers, and with [Sonic the Hedgehog] having 30-plus years of games, there are so many amazing characters. Were all dying to get a female character in the mix with Amy, but at the same time, to honor how they were both introduced in Sonic CD. So it felt very cool to honor that chronology and have them simultaneously introduced.Of course, it would be reckless and irresponsible to not have had at least some story conversations you dont want to just put a character in there and then wake up after the movies out and have no idea what to do with them.With Shadow, we knew by bringing him in at the end of [Sonic the Hedgehog 2], that was going to more or less commit us to the Sonic Adventure 2 storyline, or at least something that honored that storyline. With the movies, what we always try to do is honor the characters and stories that made them so iconic, but also give it a movie coat of paint. So now thats the challenge with Amy and Metal.I cant really speak to any of the Sonic 4 stuff, but I will just say that we never put these characters in lightly its always because we think theyre going to bring something interesting and new to the story. That really is the overall goal: to make each of these films unique. We never want to repeat ourselves. We never want to feel like were making the same movie. Its always got to take us into new territory tonally, and offer up something new, and give people a reason that they have to get out and see it. Because you dont want it to feel familiar or expected or, worst of all, predictable. So yeah, thats the next challenge.Do you have an internal wishlist of which actors might voice those characters?Absolutely. I mean, traditionally, in most of his incarnations, Metal Sonic doesnt speak. Thats not to say that once the story and scripting process begins, there wont be the question asked, What if we did give him a voice? All those conversations have yet to be had.But I think with Amy, as we work through her character when that work is committed to well be figuring out whos the best match in terms of personality. I mean, Amy is a little different from Shadow. His character has stood up over time. I think [based on] Amys original introduction, we have some different story needs [we cant just rely on] Sonic CD. But theyre both really exciting opportunities. They are creative conversations still to be had, so well just have to wait and see.Sonic the Hedgehog 3 arrives on digital rental and purchase platforms on Jan. 21.