Blizzard likes to to hide the next hero and events right in front of us, so I have 3 theories.
First theory: We will play as rein and the other crusaders/soldiers fight the omnics in Eichenwalde. (Evidence: Reinhardts Cinematic)
Second Theory: The Origin of Junkertown (and Junkertown Queen), As we know, (Evidence: Orisa came out right before Uprising and she was a major role in the event.) If she is the next hero, it is most likely going to be about the Origin of Junkertown.
Third Theory: Talon/Blackwatch during the Omnic Crisis.
(Evidence: The amount of Talon heroes we have gotten in the past)