The Flood make the most sense. In other threads, I saw that people think there are going to be Brutes and Grunts as playable characters. However, these wouldn't do because Grunts or Brutes don't have energy shields like the Elites or Spartans do. The Flood have energy shields, and resemble Elites or Spartans the most, so I think they will be in the next game.
Also, I think Bungie should make it so that you can completely customize your armor. For example:
-You should choose what type of Elite that you want to be, like an Honor Guard or a Heretic Elite. You can then customize even more with colors and emblems.
-You should also be able to work on features for your character (like the White Elite Commander from the Campaign has a part of his jaw missing).
-For Spartans, idk but maybe you can choose between Mark 5 or Mark 6 armor or something...