GTA Online stands as one of the best online games ever however will R* take it to a whole nother level with a full fledged Zombie Apocalypse DLC? Zombies and doomsday is very hot right now, and throwing Los Santos into a doomsday state with Zombies, looters, and more could launch GTA Online into the stratosphere. R* could sell a separate disc with a single player story which in turn would unlock it's own apocalypse-themed Los Santos in GTA Online.
Imagine having to scour a destroyed Los Santos in search of supplies, zombie vaccines, weapons and food while having to contend with looters and of course relentless Zombies. Zombie games do very well and are often a financial success. With the built in open world of Los Santos, Zombies could add tremendous value to the GTA franchise.