That idea was actually controversial when “GTA 4” came out, and many players were displeased with the change of tone that the franchise saw. On the other hand, “GTA 5” restored their hopes and showed that Rockstar clearly still know what they’re doing with regards to the humor of their games.
It’s possible that the studio might want to revisit their ideas from “GTA 4” and expand on them in some ways, but it’s much more likely that they will just go with the flow and make a game that people actually want to see. Meaning, it’s not that likely that the studio will reintroduce the grim setting of “GTA 4”.
“GTA 6” could actually be somewhere in the middle, on the other hand, throwing some more realistic plot lines in the mix while at the same time preserving the humorous, ridiculous nature of the “GTA” franchise as a whole. It’s important to preserve the roots of the series, after all.
According to many fans, that could actually be the best approach for Rockstar if they want to play it safe. The industry has seen a shift towards more realistic, gritty stories lately, and it could be a good move from the studio. But let’s not forget the rumors about possible VR support – a dark and depressing mood can easily be multiplied many times if the game is presented through VR technology, so Rockstar should definitely approach this part of their design with a lot of caution if they don’t want to make a huge mistake.