Well here is my idea and you can add your input. I changed a little of the story around from the movies and books in accordance to Voldemort's and Bellatrix's demise. In the book of course Molly Weasley killed Bellatrix and Voldemort is incapable of love. Well I was thinking of Bellatrix having two children with Voldemort. Yeah I know lame but really the only enemy I could see as being a worthy enemy would be a relative of Voldemort. Anyway the two children are brother and sister with the brother fully holding all of Voldemort's and Bellatrix's views on pure-blood and things of that nature while the sister starts out believing it, then becoming disillusioned, then liking one of the Potter brothers, then turning against her own brother. Anyway I would play the brother because I like evil characters lol.
Oh by the way here is a little info about the two characters. My character (the brother) knows that Voldemort was a half-blood but he lied to his sister and told her that he was pure-blood thus trying to insure that she would be as fanatical as him about "keeping the bloodline pure." Originally they were given to an orphanage after Voldemort and Bellatrix died and the sister got loving Muggle parents but the brother gets abusive, alcoholic ones thus insuring that he would have a hatred of Muggles for what his foster parents did (like Voldemort hating Muggles because his Muggle father abandoned him) and insuring that he hates any pure-blood wizards who would "degrade themselves" by mixing with Mudbloods (pure-blood traitors) and having offspring (half-bloods). Basically as I said his view is the same as Voldemort's and Bellatrix's views on bloodline and the such. Anyway when he turned 16 he ran away from his abusive foster parents and convinced the sister to join him after showing her magic and that they were related thus the sister's foster parents search for days and never find her and presume her to be dead. Thus the story starts when the sister's 14 and the brother's 16 and they get to go to their first year at Hogwarts (no one knew of their existence until then).
Brother- Me
Sister- Kopalover
(I'll come up with names for them before we start).
Albus Potter-
James Potter-
Harry Potter-
Lily Potter-
Ginny Weasley-
Hermoine Granger
Ronald Weasley-
Rose Weasley-
Neville Longbottom-
Hadgrid-
Voldemort and Bellatrix (in flashbacks and hallucinations): Bellatrix- Shadowfax
(Any other characters from the book along with OCs are also welcome)
Mia Black- Shadowfax