I was really hoping they would dive into a completely new location in 6. Doing Miami again takes away a level of excitement for me, but I'm still interested. If they did something similar to San Andreas and simulated three major cities in Florida instead of one (obviously on a larger scale) that would be pretty sweet. Miami, Orlando and Tampa could be pretty badass - especially if they also include some military bases (i.e. MacDill & Patrick) along with Cape Canaveral/Kennedy Space Center.
I played through the entire GTA5 story twice and spent a lot of time on GTA Online. I anticipate something similar with 6. One thing that would hook me even more is if my wife and I could co-op the offline story.