It's trickier than it seems, right? Because the original story is so tightly woven around its core cast and their specific, world-altering powers. Throwing an original character into the middle of that can feel forced, like a guest crashing a very intense, very dangerous family dinner.
What usually works for me is thinking about them not as a main character replacement, but as a gear that turns the existing machinery slightly differently. Maybe your OC is a merchant supplying gear to both the Military Police and the Survey Corps, giving them a unique view of the political tensions. They're not saving the day, but they're in a position to overhear plans, move supplies, or even accidentally smuggle something vital. Their story runs parallel, touching the main plot at specific pressure points without trying to shoulder the whole titan-killing burden.
Or, and this is a path less travelled, they exist entirely in the aftermath. A civilian architect brought in to help redesign Wall Rose's defenses post-breach, dealing with the trauma of the populace and the military's new paranoid logistics. Their conflict is with bureaucracy and grief, not the Beast Titan directly, but it's still deeply rooted in the show's themes. That way you're expanding the world the canon built, not just inserting a new piece onto the main board. Feels more organic to me.