A new rumor on 4Chan claims that GTA 6 will be announced on March 25. Unfortunately, there will be no trailer. Nothing big or fancy. Just an announcement on Rockstar's social media platforms in the form of a logo. The trailer has already been put together and is ready. It will come within the next few weeks.
The game will be set in Miami, after the time period of GTA: Vice City (i.e., 1986), but not modern era.
The game aims to be released on next gen consoles and PC at the same time! The release is scheduled for 2021.
A multiplayer mode will be available after the launch (similar style to GTA 5).
And another piece of information.
GTA 6 will have many features from GTA: San Andreas returned, such as fitness, hair growth, hunger, etc. A touch of realism is being added like better physics (vehicle handling, ragdoll stuff). Nothing too much, but a small step towards realism without ruining the fun. This includes food shops, gyms, hairdressers, vending machines and so on. The multiplayer will have role play functionality. It won't be based around roleplaying, but will have roleplay modes.
Enjoy the announcement. Take everything with a grain of salt. This was discussed over a few beers (probably, some Rockstar employee was talking to a friend at a bar). Nothing is finalized apart from the announcement related stuff.
To summarize, we've already heard a lot of similar information in the previous leaks. However, a simultaneous release on PC and consoles is something new. Yes, it would be great for those who don't wanna spend hundreds of dollars on expensive brand new next-gens. On the other hand, this would be weird for Rockstar to break their ancient tradition to release their games on consoles first so that Sony and Microsoft could swim in cash. Otherwise, very few people would buy console games if there is a PC version.
The time period after 1986 conflicts with other rumors which mentioned the 70s and 80s.
As for better physics, many remember that GTA 5 physics was deliberately simplified by Rockstar, comparing to GTA 4, presumably to make GTA 5 run smoothly on most PCs. We hope not only GTA 6 physics returns back to GTA 4 state but also overbids it.