Melissa or Haylee instead of Hadley. Or Jordyn for a girl. I wouldn´d use a surname as a name for a child at all because as you say it: It is a surname! There exist so many beautiful real first names. I´m sure you´ll find a good, unique and suitable one for you next baby that fits to her/his siblings names. Good luck :)
Please don't name your daughter Miller. Can you imagine the beer jokes she would get, especially since beer commercials don't exactly cast a classy light on women? NOT something I want my daughter associated with - plus, it's just not attractive.
You daughter deserves a feminine first name....as a first name, not a surname. Besides, if you want something "unique" - unisex and surnames are not the way to go, their very very trendy right now.