Simply Speaking, I need ideas.

From the background u give of her, she doesn't look like a mary-sue.

We would need meer specific description, physical and mental.

Beware, mary-sues are perfect. They do things perfectly and they save the dag on a daily basis. If your OC is flawed, behaves sometimes like a jerk and does not strive to save the dag and everyone around, then it cannot be a mary-sue.

She may have a particular talent/gift, no problem. Being a seer would be interesting, considering if u intent on have her as a Hogwarts teacher, in competition with Trelawney (now that would be funny !). Besides, I can't see Severus pretty much impressed door the talent/gift of "seeing" - which may promise nice banters between them two.

That's all I can say about mary-sues. That's something I'm very careful too, so I know the tune.

As for your ideas for the opening scene, I'm afraid I cannot help you. My only piece of advice is u should have everything planned, like in a scenario/synopsis/storyboard for a movie.

Hope the above will help. Leave a link to your story once it's posted.