A small but meaningful issue with 80's Miami again is the music. Licensing classic music is going to be prohibitively expensive for Rockstar, unless they want to blow the entire budget on music or if they could get some distribution deal with Spotify or some music provider but that's a big risk.
GTA:VC and San Andreas debuted at the sweet spot for Rockstar, as few people outside of the videogame industry really knew how profitable and how huge the market for these games was. And then you saw labels and bands pull the music out in subsequent releases of the same game.
The sound tracks of GTAIV and GTAV were both great modern sound tracks but they didn't have as many of the classic era-defining tracks that were in Vice City and San Andreas, even on the classic stations, most of the tracks were B-sides or the 3rd or 4th single release for an album (other than maybe the Seger tracks), and then the modern music they could focus on independent or lesser known bands. Rockstar still did a great job defining the essence of the game with music and the sound tracks are both great, but San Andrea and Vice City had Michael Jackson, Madonna, Guns n Roses, 2 Pac, Dre, Tom Petty, Rage Against the Machine, Ozzy, etc., and they were some of their most iconic, top of the charts tracks. A$AP Rocky and Chaingang are fine and their tracks are great, but they don't really compare... Modern pop stations aren't playing Hall & Oates (as much as I like Hall and Oates)
I'd love to see a return to the retro 1980s because it's such a better time for Miami and telling a videogame story. I think the limits of the world (e.g., no handheld cell phones) make for challenges that would make the game more interesting and force them to handle interactions throughout the world in a more creative way than just relying on the cell phone as your hub to do everything.
Also, I'd like to introduce more ownership into the world, which was improved upon with GTAV (from IV), but still a step back from GTA:SA. GTA Online's ownership model is pretty good, but GTA Online kinda sucks unless you're really into it and enjoy grinding or exploiting. Specifically, the car ownership model in GTA Online -- while it was rough for the first six months -- has really been improved and the insurance system as a soft-threat to not destroy your vehicles works.
If in Vice City, I'd like to recreate the Cocaine Cowboys, Tony Montana, take-over-the-city mentality with at least one of your characters (assuming there's multiple), turning him/her into a legitimate Vice kingpin and "owning" major portions of the city. GTAIV and V both went in the opposite direction, to sort of play a trick on the player setting them up that they're going to become these powerful players, but ultimately, that doesn't happen and it's all a "lol American dream is shit" parable over and over again. I get that's the moral that Rockstar wants to tell you, and they've done so over ~200 hours of the last two games (and another ~100 hours of RDR), so I'm hoping they challenge themselves to not tell the same story three games in a row, and consider letting the player do what they want to do in terms of city and world building like they did with Vice City and San Andreas (San Andreas especially). With multiple characters, I'd like to see one character representing the law, either as a semi-straight/somewhat-crooked cop, and another representing the criminal element, having their stories intertwine and work... Not purely antagonistic, but not purely accommodating either.