Even though it's not even out yet, ‘Sonic the Hedgehog 2’ may very well have already started hinting at Metal Sonic as the final boss of the trilogy and Robotnik's ultimate endgame. Starting with 2020's ‘Sonic the Hedgehog’, Dr Robotnik seemed uniquely invested in harnessing Sonic's power for himself. Traditionally, Dr Robotnik's interest in Sonic has always ended with his plan to kill Sonic so he'll stop ruining Robotnik's plans. One of the schemes meant to get Sonic out of the way was Metal Sonic. While the motivation in the movies may have started out differently, it would make sense if Robotnik were to finally harness that power and create Metal Sonic for revenge.
Metal Sonic was always a staple of the Sonic the Hedgehog game franchise. A robot duplicate of Sonic, Robotnik created him to be superior to the original. He's appeared in a number of mainline Sonic the Hedgehog games and was even the main antagonist of 2003's Sonic Heroes. In that game, Metal Sonic rebelled against Robotnik out of his desire to prove his superiority. There it took nothing less than Super Sonic and several of his friends to bring down, proving Metal Sonic to be one of Robotnik's most powerful and dangerous creations. So it wouldn't be a surprise if the film franchise really is hinting at his being the ultimate goal of Jim Carrey's Robotnik.
When the first Sonic the Hedgehog movie raced in and surprised everyone, it also established a motivation for Dr. Robotnik that seems to suggest the Blue Blur might eventually end up with a doppelgänger. The first movie largely did its own thing, not borrowing from any particular game or comic storyline.
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