I know Jersey was sorta done in IV, but it wasn't done properly.
New Jersey (Alderney) would be perfect for a GTA game of this gen as it's a pretty small state that's got a huge variety of of sights and places to see. North Jersey is comprised of rich suburbs and run down urban areas (Newark and East Orange are notoriously crime ridden) juxtaposing some of the wealthiest and poorest areas in the country in one convenient location. North Jersey is also extremely close to New York City, with the NYC skyline being visible from many towns, which would be a perfect connection to GTA's Liberty City. Let's also not forget that The Sopranos, one of the highest rated TV shows of all time was based in North Jersey, which we can be sure Rockstar would be able to make great use of.
Central Jersey is one of the wealthiest areas in the country and home to several attractions such as Six Flags, the now infamous Jersey Shore (part of it anyway), and several famous racetracks. Central Jersey is also home to several of Jersey's more well known colleges. There's also tons of shopping malls and historic landmarks waiting to be spoofed.
South Jersey of course, is known for Atlantic City, which would be a prime location to be explored by a GTA game, as it's pretty much a seedier, dirtier, down-scaled version of Las Vegas with a huge history of organized crime. On the other side of the region lies Camden, New Jersey, a city that is constantly jockeying for the title of "America's most dangerous city", Camden is both an economic hub and a cesspool of gangs, drugs, auto-theft and one of the highest murder rates in the country. It also shares a border with the city of Philidelphia, the perfect segway to a famous American city never explored by a GTA game.
I know Jersey's sorta been half assedly done before, but I'd love to see it done proper. If not in a full GTA game, perhaps an expansion to GTA Online.