I think it should be as realistic as the series has been for the most part. The story, characters and dialogue should be extremely realistic, while the company/brand names, slogans and ads should be ridiculously unrealistic. Being wacky and over the top worked great for Saint's Row 3, but it's just not GTA.
I hope the fake slogans are more creative than what we've seen so far, I mean, "Putting the FU in fumigate"? Usually they have a double-meaning, but that one didn't. It just didn't make any sense.