Wow, I didn't think this idea would get so many negative comments, but its cool; I did ask for opinions after all lol. Anyway yeah zombies has been overused in many games but if Rockstar were ever to use zombies, I think they would do it right in GTAV. Well thanks guys for all the replies and comments, I'm glad to see that these forums are alive and well; unlike most forums I've joined. I honestly didn't expect this much replies.
I do wish that we had more of a discussion. The vast majority of responses had little to no thought put into them. Perhaps I have become the somewhat informed discussion on Reddit where troll/asshole responses tend to disappear. I just want an add with the ability to replay it and have different experiences. I think a survival/zombie/alien invasion type thing would create that if they added the ability to create clans and bases and such. I think of games like Skyrim, where you can play the story 500 different ways and get a different experience based on how your build your character.