Facts. I like to balance things out when I write stuff. Maybe it'll start off with a villain who is super powerful that you need to grow stronger than to defeat, then next a villain you need to outsmart, then a villain with a great arsenal of abilities for their verse, etc. It keeps people guessing and it isn't the same thing over and over. Villains that also represent ideas and philosophies are also interesting to see, because you can't defeat an idea or a philosophy by punching it in the face, you defeat it by proving it wrong and bringing people into the light, so to speak.
Also, with ki control, I'd personally make it that it still has immensely catastrophic effects, even when an individual is suppressed. For instance, if someone's up against an opponent that has no control, or simply isn't using it for the purpose of creating destruction, then it'd still have unprecedented side effects. I'm surprised no villain in Dragon Ball has ever come up with that idea, tbh. Then again, it is a show where everybody lets everybody else transform (with the exception of Android 17), only to immediately regret it later.