He definitely was, but I think the writing team could just as easily make him more villainous. Pete is laughable, regardless of how they characterize him in KH3. They screwed him over in KH2, BBS, and coded (and Days...) Do you think the audience would take anything in Arendelle seriously if Pete were the villain?
Maleficent would be better but not by much. What made Arendelle interesting to me was the way they portrayed magic and the fear surrounding it. What could Maleficent possibly do to help the story? Why would she want Arendelle? Is she trying to manipulate Elsa? That could work, but what is her motivation? How would this tie into the game's overall story?
A Darkness is definitely the best choice, if only because I can see a deeper level of manipulation of Elsa going on. Because of her fear, somebody like Young Xehanort, Saix, or Vanitas could scare her into staying away from Anna and Arendelle. But again, what would be the motive? Elsa's magic in tandem with planting the seed of doubt in her heart?