If there’s a sequel to The Incredibles, I feel like the villain won’t be The Underminer despite some people’s posts about waiting for a sequel and posting a photo of The Underminer (as if they’re waiting for a movie about the family battling him). He seemed like a minor villain, like just one of many the Incredibles will defeat. Based on his short screen time, his character didn’t seem very developed. To me, his one-liner showed that he was probably a generic villain.
My point is, I don’t think he will play a big part in the sequel, if he appears even. If he will be in the sequel, I’m sure they’ll have added dimensions to his character.