In the new book The Star Wars Archives: Episodes I-III 1999-2005, Lucas shares his original intentions to make Darth Maul the central villain of the sequel trilogy.
Essentially, Darth Maul takes on a young female apprentice named Darth Talon (who appears in the comic books).
Darth Maul and Darth Talon become “the two main villains of the trilogy.”
According to Lucas, Leia was also going to have a much larger role in the sequel trilogy.