Alright, so I see this question pop up a lot, and I think people tend to overcomplicate it immediately by thinking about specific power sets or flashy weaknesses. The real foundation that gets skipped is character motivation. I've written a few OCs for 'My Hero Academia' fics, and the ones that feel flat are always the ones where I designed the Quirk first and tried to graft a personality onto it.
What ended up working was starting with a simple, almost mundane personal flaw that has nothing to do with the Quirk itself. My current OC has a sensory-based Quirk that's objectively strong in a fight, but her core flaw is a deep-seated, irrational fear of hospitals. It sounds silly, but it forces narrative consequences—she'll avoid necessary medical care, she'll lie about injuries, she panics in certain settings. That flaw creates more interesting tension than just 'her power gives her migraines' because it affects her relationships and decision-making on a human level, not just a tactical one.
From there, I tied the power's limitation directly to that emotional core. Her Quirk's overuse doesn't just drain her stamina; it literally causes sensory deprivation, making the world go quiet and gray. For someone who relies on her senses for safety and connection, that's a terrifying psychological price to pay. The strength and the weakness are two sides of the same coin, and neither feels tacked-on. The balancing act happens in the story, not on a spec sheet, because she has to constantly choose between being effective and safeguarding her own sense of self.