If Metal Sonic really will debut after Sonic the Hedgehog 2, then everything's already in place to make him an even better villain than Dr. Robotnik. The hotly anticipated Sonic the Hedgehog sequel is already borrowing heavily from a plotline that could be used to establish a precursor to the deadly machine, as it is. If that happens and fails, then Robotnik's short temper and habit of belittling others could come back to bite him, as it usually does. A defeat faced in the second movie could immediately make Metal Sonic a far more compelling character than the man who created him.
Robotnik isn't traditionally known for being the deepest villain. Comics aside, he's typically just a megalomaniacal egomaniac who's fairly transparently evil. The first film does virtually nothing to deviate from that. If anything, it dials up some of those tendencies to eleven, doubling down on them. It makes for a memorably entertaining performance from Jim Carrey, but nothing that helps to elevate the character. The most real information the audience is given about him is that he's an orphan. Unfortunately, that information doesn't really inform anything about his character or his actions as that character. Metal Sonic, however, has a path clearly paved to help him avoid this issue.
Starting with Sonic the Hedgehog 2, it's already clear where the storyline comes from. More than anything, it's borrowing from Sonic 2, Sonic 3, and Sonic & Knuckles. One thing that the former and latter have in common is a particular Robotnik creation - Mecha Sonic. In the end, Sonic and Knuckles put aside their differences to fight Robotnik and this updated creation of his. What's even more worthy of note is that the two appearances have distinct designs that suggest upgrades were made between games. The order would be different. Sonic 3 and Sonic & Knuckles both take place after Metal Sonic's debut in Sonic CD. However, for the purposes of having one cohesive narrative, it would make sense if these Mecha Sonic upgrades eventually lead to the far superior Metal Sonic.
Still, going through these upgrades would mean that Mecha Sonic would have to fail in its original mission, possibly more than once. Robotnik has a tendency to be short-tempered and belittling. It's especially true of Jim Carrey's Robotnik. There's a documented history of Robotnik berating his creations when they fail, then upgrading them. In fact, this is the entire storyline of E-102 Gamma in the first Sonic Adventure game. If that trait carries over into the films, it could be done with Mecha Sonic as Robotnik begins to grow tired of losing to the blue hedgehog. After Mecha Sonic loses to Sonic, possibly with an assist from Knuckles, Robotnik could slink away with the remains and salvage them while lashing out at his sentient creation.
Typically, Metal Sonic is an obedient and silent servant to Robotnik. He's menacing but fairly one-dimensional. That doesn't need to be the case here, however. Combining his humiliation by Sonic and Knuckles in Sonic 2, combined with his abuse by Robotnik, could give Metal Sonic all the personal motivation he needs. A genuinely forged hatred for both Sonic and Robotnik could compel him to obsessively prove his superiority to the former and later betray the latter. After all, while it's not particularly common, Metal Sonic has been known to act on his own in the past and even betray him on occasion when his AI outgrows the doctor.
Robotnik gets remarkably little in the way of motivation in the films. He's simply driven by ego. Yet while Metal Sonic is usually driven purely by his programming, that doesn't need to be the case in the films. A deeper motivation, driven by his failures as Mecha Sonic and his master's treatment of him, could be everything he needs to surpass Robotnik as a character and a threat in a seemingly inevitable third movie. For now, though, it all depends on what happens in Sonic the Hedgehog 2.
