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. Based on the trailer for Sonic the Hedgehog 2, however, and the antagonistic relationship between Sonic and Knuckles it sets up, it appears that the movie will be borrowing heavily from three different games: Sonic 2, Sonic 3, and Sonic & Knuckles. All of these were pre-Sonic Adventure era games. Sonic 2 introduced Tails. Sonic 3 and Sonic & Knuckles were essentially two halves of the same game, the first one introducing Knuckles and the second making him playable. Both Sonic 2 and Sonic 3 also introduce the first robot duplicates of the blue hedgehog, Mecha Sonic. Assuming the movie franchise continues with that time period, that only leaves one more game to adapt for a third Sonic the Hedgehog movie: Sonic CD, the first appearance of Metal Sonic.
Metal Sonic wasn't the only character to make their debut in Sonic CD, however. As early as the release of the first film, rumors began to swirl about the addition of Amy Rose to the franchise. Amy Rose was a character who made her video game debut right alongside Metal Sonic, being kidnapped by the robotic version of Sonic while following the hedgehog. In order to rescue Amy, Sonic had to beat Metal Sonic in a race while also dodging Robotnik. A scenario similar to this could easily work for the climax of a Sonic the Hedgehog 3 movie, or even a mid-point, given Metal Sonic's tendency to betray Robotnik.
All roads have pointed at Metal Sonic being the antagonist in the final installment of this trilogy for some time. Sonic the Hedgehog set up Robotnik's interest in Sonic's power. The sequel uses up all of the other games from that era for inspiration. Even the rumors of Amy's joining heavily implied that Metal Sonic could be on the way. The only thing left would be seeing how the filmmakers chose to bring him to life. Tails was teased at the end of Sonic the Hedgehog, back in 2020. Perhaps Sonic the Hedgehog 2 could tease the Blue Blur's metal copycat.
