Jeb and Christie together make it much harder for some of the fringe candidates (Huckabee, Santorum, Perry, Cruz, Carson, Palin) to break through. Had Mitt joined the field, the establishment vote and donors would have been split three ways. Christie seems determined to compete in Iowa while Jeb can skip it and avoid a potential embarrassing loss or being forced to the right on social issues. Establishment can keep Graham afloat through SC to avoid somebody getting on a roll (Iowa, NH, SC) before Florida.
Walker or Kasich would expand the electoral map, putting blue states (WI, MI, PA) and swing states (OH, IN, IA, MO) into play. Jeb with Martinez or Sandoval as VP would work, but Kasich or Walker might work if the GOP decides to give up on Latinos altogether.