With Sonic’s world in the film appearing to be on an island and Robotnik learning inklings of the power Sonic has by himself, it would be very easy for a sequel to set up the existence of the Chaos Emeralds and the Master Emerald. Knuckles’ first appearance in the games saw him team up with Robotnik after being tricked into thinking that Sonic is trying to steal the Master Emerald, and though that explanation wouldn’t work in an immediate sequel, given Knuckles’ tribe is after Sonic and his powers, it would be just as easy for the echidna to travel to the mushroom planet in search of Sonic and find Eggman with Sonic’s quill.
Well, the start of the film saw Sonic come to our world to escape a tribe of echidnas who sought to use his powers for their own means.
The end of the film sees Sonic and Dr. Ivo Robotnik engage in a globe-trotting fight, going from The Great Wall in China to the pyramids of Egypt before ending up back at Green Hills, Montana.