Watching Harry Potter and the Goblet of Fire on TV, I hatched upon a design theme I would include if I got to work on an HP game.
Basically, it's: relationships are a lot of work, sometimes awfully so, but if you can get it, love is very much worth it. In magical terms, it lets you break the rules once in a while. Neat! Dark wizards, naturally, are folks who have no more love in their lives, or who forsake love at just the wrong moment for the sake of ambition. Thus, they have to become extra-ruthless and clever with their magic (and with other things) in order to compensate.
Alternately, I would be quite intrigued by a game of My Life With Master, hacked to use HP as the setting and premise, wherein the protagonists are all Dark wizards serving You-Know-Who. The identity of the Master should be obvious ^_^