In many of these fanfictions, a common theme is redemption. Even though Harry is the 'Dark Lord', there can be a journey towards redemption. This could involve him realizing the error of his ways after causing chaos in the wizarding world. Another theme is the exploration of the Slytherin side of Harry. Since Slytherin is often associated with dark magic and ambition, it gives room for fanfiction writers to play with the idea of Harry embracing his Slytherin traits more fully. Love, in a twisted way, can also be a theme. Maybe Harry's new dark self still has feelings for Ginny, but they are expressed in a much darker and more complex manner.
The theme of alternate universes is quite prevalent. In these 'Dark Lord Harry Potter' slash fanfictions, the idea that Harry's fall to the dark side creates a completely different wizarding world is fascinating. It could be a world where Voldemort never existed, or one where the Dark Arts are the norm. Another theme is the moral ambiguity. As Harry becomes the 'Dark Lord', the lines between good and evil are blurred. Fanfiction writers can delve into how characters like Dumbledore, McGonagall, etc., react to this new Harry who is neither clearly good nor bad but exists in a gray area.