Question about writing my story?will choose best answer!?

Okay so I want to write a novel about mermaids. But they need an opponent and I can't figure out if want to use vampires or wolve! I know both are overly used in today's literature but i am going to shed em in a new light promise! I want it to be about her mermaids, who dominate the sea, and wolves/vamps who dominate the land. But I can't decide who I want the mermaids big enemy to me:( help!