The May 9, 2019 This Week on Xbox video seems to have revealed a new character for Dragon Ball FighterZ. During a portion of the presentation talking today being Goku Day (as in 5/9, go and ku), it noted you could get GT Goku and Janemba fighter skins for the game. However, Janemba is not currently in the game. (Thanks, Resetera!)
As of right now, one character in the Dragon Ball Fighter Z second character season pass has not yet been revealed. However, the Dragon Ball Z: Fusion Reborn villain has been discussed as a possible inclusion online for the past few months. However, there has been no confirmation. Also, beyond this Xbox presentation, Janemba has not been named.
Here is the This Week on Xbox presentation that mentions Dragon Ball FighterZ and the Janemba skin.
Dragon Ball Z: Fusion Reborn was a 1995 movie in which Janemba is a monster capable of altering reality. He also appeared in the Super Dragon Ball Heroes, Dragon Ball Xenoverse 2, and Dragon Ball Fusions games. (The screenshot above is taken from his Dragon Ball Xenoverse 2 appearance.)
Dragon Ball FighterZ is available for the Nintendo Switch, PlayStation 4, Xbox One, and PC.