Oof, this takes me back. Saw a lot of 'hidden leaf princess' or 'Uchiha long-lost sister' types when the fandom was hot, and they never stuck. The thing that makes an OC work is how she grinds against the world's rules, not how she slots into an existing power vacuum.
I'd start with a flaw that Konoha's system would actually punish. Maybe she's from a civilian family that resents ninja privilege, so she's cynical about the 'will of fire' stuff but forced to play along to get by. Her motivation shouldn't be to date Sasuke or rival Sakura; it's about surviving in a system that sees her as expendable. Give her a practical, unglamorous jutsu specialization—barrier techniques or trap-making, something that emphasizes prep over raw power. Makes her fights more about cleverness.
What sells her is the small moments. How does she react when a mission goes wrong and a teammate is hurt? Does she follow protocol or break it? That conflict between personal ethics and shinobi code is where she becomes real, not in which canon character notices her.