I’ve heard several other Democrats mentioned, all with pretty low voter recognition. One is Amy Klobuchar. If they were to nominate her, they’d have to count on the Republicans nominating someone with a name even harder to spell. Maybe Jarrod Saltalamacchia (if he is old enough), not a Walker or Bush or Cruz or Perry.
Just checked—Jarrod will be 31 in 2016, so the Republicans would have to pick someone else.