Patrick Murphy has the right profile to run statewide, representing a district that is somewhat more Republican than the state as a whole is not a bad staging point for a statewide campaign.
The mayor of Jacksonville might be an option (depending on how good a job he's done as mayor), as well as Val Demings.
Alvin Brown could never win the Democratic primary; he's far too right wing for the urban leftists. It'll get him a pretty easy reelection, but he'll never fly with the state Dems unless they're even more desperate than normal (which is already pretty desperate).