Otherkin

This story is never going to be finished, so I'll just tell you what happens.

Serena teaches Harry Occlumency, so Voldemort can't influence him. This puts him back on good terms with Dumbledore, and together Serena and Dumbledore discredit Umbridge and get Remus reinstated as DADA professor. Snape hates Serena even more, since he still wants the DADA job. Serena helps Remus teach the class because she's so brilliant at defense.

In fact, she's so brilliant at defense that she goes out on her own and defeats Voldemort, rendering all the other characters useless.

She also finds time to give Hermione a Gawth makeover, turn Draco "good," and set Hermione up with Harry so she can have Draco, or set Hermione up with Draco so she can have Harry - I never quite decided.

Then she starts getting them all to form a rock band, but Snape has finally had enough and kills her in her sleep. The end.