The optimal warband is something that will most likely remain mostly the same in-terms of wanting each class because of how the game is designed. There are 8 spots available in a warband, and there are 6 classes in Diablo Immortal on launch, which means 6 of the 8 spots are pretty logical. You should strive to have at least 1 of each class because this will allow your group to evolve with the game and have what you need for different situations in all forms of content.

This means after having a Barbarian, Crusader, Demon Hunter, Monk, Necromancer, and Wizard, we are left with 2 remaining slots in our warband as flex spots. The game is designed in a way where half of the classes are primarily melee, and the other half are primarily ranged. This means we can stay balanced by splitting these 2 flex spots between 1 melee, and 1 ranged class. So for our first flex spot this means choosing either another Barbarian, Crusader, or Monk, and for our second flex spot this means choosing either another Demon Hunter, Necromancer, or Wizard. I think as of now there are no terrible choices for these last 2 flex spots as we will be learning a ton about the game in the near future, and Blizzard will surely strive to balance things in a way that all classes will be included. If I were forced to pick one optimal warband set-up to recommend it would be filling the last 2 flex spots with Crusader and Demon Hunter, resulting in a warband that looks like this.