@Whether these games are dark or not, IMO the III trilogy and their 3 respective handheld prequels were more of a dark comedy; they displayed some gruesome or reprehensible acts but went about in a ridiculous and over-the-top way in it's execution (see "Deconstruction" in SA or delivering drugs in a cartoonishly loud Mr. Whoopee truck in any game), and the dark topics discussed (drug addiction, death, regrets) were all covered in irony and/or jokes that elevated the tone from ordinarily heavy to light-hearted.
I don't think they were all that intentionally focused on being dark for that matter, 9/11 changed the way the media could be edgy about topics and R* were more concerned about GTA being fun and chaotic tributes to their favorite movies and music from a set time period as far as the III era's concerned.
Hell, I don't completely buy IV being "the dark/serious one" either. It certainly took itself more seriously, but it's near impossible to take a game where a criminal feels bad and depressed as he inflicts untold damage to a city and its denizens seriously. I think R* knew it too, hence TBoGT going back to the III/2 era tone.
The biggest thing I would want from GTA 6 is they nerf the fucking cops. The games normally had a survivable three star
IDK, replaying III and VC and unless you cheese it by exploiting collision models and certain spots, those 3 stars will become 4 real quick as you mow cops down to avoid getting run over. VC cops like to spawn feet from you at every corner and III's cops were near jihad in their efforts to waste you, forget busting you after 2 stars.
That said I agree V cops were a knee-jerk reaction to their pathetic nature in IV. Having a squad full of cops show up after I ice some NPC in Chilliad is retarded. Even the III era games would spawn 1-2 cops at a high wanted level or wait until you went back to the roads before unleashing the FBI on your ass.