I'm going to split this into two discussions: Villains of Hank Pym and villains of Scott Lang. Pym's villains would work best in a prequel, imo, but there's still room for someone from the past coming back. But for Scott Lang, I think an obvious choice for a sequel's villain would be the Taskmaster. I know he's sort of a mercenary of the Marvel Universe and has fought a lot of people, but he seems more prominent than most when it comes to someone Lang has fought. I guess the only question is whether you can get a cool battle between someone who shrinks tiny and someone who doesn't.