well, it would have to be serious. no bigfoot,chupicabra, etc. it would have to be straight up guns, cars, and zombies. honestly, GTA 4 would have made more sense to put the mode in, because the west coast is all sun and fun when it comes to mind. but i still think a zombie mode would be a pretty good idea for people who are into that sorta stuff, which nowadays is ALOT of people. look at the other zombie games that have made alot of money. Not to count movies,books,guns/equipment, that have made companys lots of money. GTA has a realy good base for a zombie mode, givien a few tweeks to the gameplay. Just an opinion
Rockstar doesn't need to do what's popular to get money. They start the trends, not follow them.