1) Orm as the antagonist.  If you look at the situations in the ocean in our own reality and it’s declining state from a realist perspective, Orm appears as a well-adjusted person trying to avert the apocalypse.  This guy is going to behave like the Americans in your typical alien invader movies.  Except that he’s attacking Americans.  So we get to see that kind of attitude from the receiving end.  
2) Orm as Aquaman’s villain.  While this might seem redundant, this is about Orm vs. Arthur as opposed to Loki vs. Thor.  Given Loki’s appeal, DC needs to compete with that and the only sibling rivalry in the Justice League movie lineup is arguably Aquaman and Orm.  
3) Where to fight.  This will be a factor, if not THE factor that really holds people.  People who liked “Man of Steel” liked the battles in Metropolis, though critics and displeased viewers panned Superman’s mass manslaughter and delibe