The first question is who will run on the Democratic side. The issue Democrats face is that they have no obvious candidate. That said, John Wynne of Carolina Strategic Analysis provides a list of people whose name have come up as possibilities:

Dan Blue, NC Senate Minority Leader
Janet Cowell, State Treasurer
Cal Cunningham, former State Senator and U.S. Senate candidate, 2010
Anthony Foxx, U.S. Transportation Secretary and former mayor of Charlotte
Larry Hall, NC House Minority Leader
Kay Hagan, outgoing U.S. Senator
Allen Joines, mayor of Winston-Salem
Mike McIntyre, outgoing U.S. Representative
Charles Meeker, former mayor of Raleigh
Brad Miller, former U.S. Representative
Heath Shuler, former U.S. Representative
Allen Thomas, mayor of Greenville