One idea could be that the Overwatch heroes are transported to the Attack on Titan world. Tracer could use her time - traveling abilities to outmaneuver the Titans. Maybe Reinhardt's shield would be crucial in protecting the Survey Corps from Titan attacks.
Another possibility is that the Titans in the Attack on Titan universe start to appear in the Overwatch world. The Overwatch agents would have to adapt their combat strategies. For example, Widowmaker could use her long - range sniping skills to take out Titans from a safe distance, while Mercy would be busy healing the injured agents during the battles.