It appears that Rockstar Games could be moving in earnest toward a GTA 6 reveal: Earlier this month, hip hop group City Morgue shared an image on Instagram suggesting that it was involved in a partnership with the studio that would be revealed next summer.
City Morgue, a hip hop group, teased this on Instagram today pic.twitter.com/SgmuT6jAK7November 10, 2019
There's no direct reference to the series, but music is a major part of the GTA series and while I'm not familiar with City Morgue's work, I think it's safe to say that it'd be a good fit for a radio station or two. It's possible that the tweet is related to a new Bully game instead, although that seems like a bit more of a stretch, as does the possibility of a Bully sequel in general.
As noted last week by PlayStation Universe (via GamesRadar), Rockstar recently posted a job listing for a animation systems engineer, "to help us in developing advanced animation systems for large open world character-based games." That description could be fairly applied to other Rockstar games, though, including Grand Theft Auto 5, which despite being six years old is still tremendously popular, and maintain a solid grip in the top ten most popular games on Steam.
Rockstar has collaborated with numerous musicians in GTA5 previously—Frank Ocean got his own personal radio station—and it's possible that this could be a something similar to last year's appearance of Tale of Us, Dixon, Solomun, and The Black Madonna in GTA Online. But the temporal distance makes me think that this could be something bigger. The Black Madonna teased the "summer 2018" GTA Online appearance just a couple of months before it happened, not half a year.
Whatever's going on, it does appear to be serious: Via redditor Wardoganthem187, City Morgue said that more information would be coming "soon." Steven Ogg, who voiced Trevor Philips in GTA5, said the same thing during the recent Brazil Game Show, and with new console hardware expected to arrive at the end of 2020, a mid-year reveal of a next-gen GTA wouldn't surprise me at all.