Honestly, what trips up most OCs is trying to be 'important' to the main plot from the jump. Makes them feel like a tourist in the story, waving at canon characters. I figured out my favorite OCs are just people who already existed in that world, doing their own thing that the plot then bumps into. Like, I wrote a witch who worked in the Department of Mysteries' Time Room archives. Her whole deal was cataloging paradoxes, she was fussy and hated disruptions. When Harry's lot inevitably blow something up, she’s not a hero—she’s the exasperated bureaucrat filing the incident report, which accidentally contains a clue. Gives her a reason to interact, a unique lens on the magic system, and keeps her from being the Chosen One’s new best friend.
Power scaling is another pitfall. Giving them a super-rare magical affinity or being a secret Black family heir often reads as wish-fulfillment. Constraint breeds creativity. Maybe their magic is average, but they’re brilliant at arithmancy, using it to optimize potion recipes for St. Mungo’s. Their strength isn’t in power, but in a specific, grounded expertise that offers a service to the narrative. Makes them useful, not a spotlight-stealer. The world needs more competent side characters, not another prophecy kid.