Chaos
I think there’s a strong possibility that Chaos, the water-like mutant antagonist of the original Sonic Adventure, is the villain in a future Sonic movie. Whether that’s Sonic 4 or another movie down the line, Chaos is the kind of world-ending threat that these movies seem to be leading toward as they escalate and raise the stakes each time. However, I do think these movies need a marketable face to tease what’s next, and that’s why Tails and Shadow’s appearances at the end of the first two movies made sense. They were both well-known characters that you could see facilitating merchandise like figures and plushies. Chaos is exciting to long-time Sonic fans and I would love to see him flood a city and Sonic make some joke about how the team is “gonna need a bigger boat,” but as a post-credits stinger? There are probably more likely candidates.
