I've only played 3, VC and SA and have 4 on my very long queue.
GTA5 looks 'pretty', but from what I've seen its lost alot of the heart & soul of the earlier (And dare-I-say better) entries.
In VC & SA, I missed the original music on the radio stations in 3. I felt it not only helped fully flesh-out the satirical elements of the world and gameplay but also made that world something more unique and authentic on its own right. I already have my 80's music favs imprinted to other memories thank-you-very-much.
I've been waiting to see if Rockstar would realize what a massive potential they have in the GTA IP and just do something radical with it or at least spin it off.
Think: A modern conventional RPG/Sim-like counterpoint to something like the later gen Final Fantasy games in a fully explorable city. You could go more along the sim route and have a character(s) with a selectable conventional job and that job's 'winning' endpoint or just being a small-time scumbag and working your way up through the gangs and the syndicates into the role of a 'respectable' legitimate elected official.
And by 'more' sim, I mean ALLOUT sim. Include the minutiae of buraeucratic laws, rules and regulations and BS that makes everyday life so much fun, but with an outraegeous and GTA-esque slant. *g*
What I'm imagining would only be palatable to developers as a online-only MMORPG though.
Also: An online graphically 1st person explorable representation of a city (Think New York, Paris or London) with storefronts (Taco Bell's, Office Max's, Best Buy's etc.) paid for by their respective owners ala a sort of ingame advertisement, but also maybe linkable to ingame/outofgame purchases? Maybe other faux storefronts businesses can purchase for a very small fee to get their signage in the game. Heh, contemporary class ideas: gangster, thug, cop (paladin equiv?), bumster, student, knackster (an evil geek), politician (wizard or lord equiv?). A few of those are some of the 'anti' classes of course.
Just a thought/idea. I'm about 80% retrogamer anyways. These plastic/cartoony/gritless and scripted games from the later generations of developers don't appeal to me.