Astorian nailed it, and I’d add that Clinton values loyalty. She will pick someone she has a long relationship with and who has supported her over the years.
I see these as the frontrunners:
Tom Vilsack
Terry Mac
Wesley Clark
I see these as dark horses:
Erskine Bowles
Bill Richardson
Howard Dean
It will not be Julian Castro. Just stop, people. And media. Hillary Clinton can count, and she lost in 2008 for failure to understand the process. She has to know that the Latino vote will not decide the 2016 election. It’s going to be all about women and African-Americans, so if she does make a direct play for a demographic, it’s going to be Cory Booker or Deval Patrick. And if she did want a Latino she’d just pick Richardson(assuming he could pass vetting), who is ready to be President on Day 1 and is a longtime Clinton loyalist.