Analyzing prior games, there should be an archer and a holy warrior. These roles were filled by Amazon and Paladin in D2, and Demon Hunter and Monk in D3.
My guess is Assassin will be reworked to fill the archer role.
My initial gut check would be seeing a Cleric, focused on shield play, holy smiting, and an angelic warrior focused on El’Druin or Al’maiesh.
Edit: The more I thought about this and read prior comments, I believe some form of cultist, that uses angelic power and death magic will be the fifth class. The necro paladin hybrid. The REAPER.
2nd edit: Which all classes first initial is an acronym Easter Egg for the first DLC class.