Datamining turned up
three codenames for upcoming characters in the games params:
fighter_kind_jack
fighter_kind_brave
fighter_kind_packu
"Packu" being a Japanese onomatopoeic for biting, it stands to reason that that's Piranha plant. "Jack" is likely Joker, either referring to the playing card theme or Atlus' mascot Jack Frost.
"Brave" is the one that's got people stumped. The most convincing theory I've seen is that the Japanese name of the Hero class in Dragon Quest 3, yuusha (勇者), translates to "brave." The only character in DQ3 that can be the Hero class is Erdrick, so this might confirm that Erdrick's next up after all.