It could be Voldemort. Even though the Darth Nox element is introduced, Voldemort might see Harry's new connection as a threat to his own power. He could try to either use Harry's new powers for his own gain or destroy Harry to prevent him from becoming too powerful with this strange new heritage.
The main antagonist could be an ancient evil that has been awakened by the discovery of the Heir of Darth Nox. This evil might be something that predates both Voldemort and the Sith. It sees the combination of magic from both universes as a way to finally achieve its goal of total chaos. Harry, as the one connected to Darth Nox's heir, has to stop this ancient evil before it destroys both the wizarding world and any other world it can reach.
Perhaps a new character who is also vying for the title of Heir of Darth Nox. This character could be from the wizarding world but has been secretly studying the ways of Darth Nox. They see Harry as an obstacle in their path to total domination, using the dark powers they believe are rightfully theirs. This would lead to a battle of wills and powers between Harry and this new antagonist.