The same thing Superman always dreamed of... a better world.
There is little I can say about Injustice: Year 3 #13 that is not hinted at by its title - For The Man Who's Lost Everything. With a story centering around Superman trapped in a dream, comparisons to the classic Alan Moore story For The Man Who Has Everything are inevitable. One wonders though - especially after the powerful ending of this issue - is Superman the titular man? Or is it someone else?
The artwork by Bruno Redondo, Juan Albarran, Xermanico & Rex Lokus does a fine job of conveying the story. More, it is filled with lots of novel little touches. I particularly like Harley Quin biting her lip as she realizes just how screwed she and The Joker are on a scale of 1 to 10.