MultiVersus official Twitter account confirmed this weeks update will add Black Adam and Arcade Mode.
Unfortunately, no additional information was given on Black Adam or Arcade Mode, though data miners have added some insight into Arcade Mode. According to those reports, it will be a 10 match gauntlet that concludes with a fight against your selected characters rival. Most of these are simply another character from their universe, like Velma and Shaggy or Jake and Finn, or relate to the character in question in some way.
Expect more details in the next couple of days when the full patch notes are released.