Since World War Z dealt mostly with Pitt's Gerry Lane traveling around the world looking for clues, we didn't get many opportunities to see the affects the zombie breakout had on society. By doing a sequel, it would give the filmmakers a chance to show how humanity adapts to this new world. At the end of the film, we do see the beginnings of our fight back against the zombies (the military bombs a football stadium filled with the infected and people fight back after they've been injected with the vaccine), which could be a main story thread for the sequel. For a movie titled World War Z, there wasn't much warfare going on. A second installment to the franchise could amend this and deliver a film with the scale of a world war with epic battle sequences. Good sequels always expand upon their universe and show audiences something new. By diving into the unexplored plot element of how Earth has changed as humans attempt to exist in a world with zombies, the filmmakers could show us something interesting as they continue the series.