It will be in a near-future setting, while not so far off like the 2070s in Overwatch, it's safe to assume that it will be set probably anywhere from a few years to a decade ahead of the game's release.
it will release in 2020 on consoles, 2022 on PC, with the PC version coming with VAC support, 64-128 player sessions and a custom map creator ala Far Cry 4 so you can make maps that can be played on private sessions with crew members and/or friends. You can upload your maps and browse those made by other players on Rockstar Social Club.
it will either be set in Vice City and its surrounding areas or the GTA universe's equivalent of Chicago (Anywhere City?) and Detroit (Carcer City?) and their surrounding areas. Probably the latter since one could distort the latter to include mountains and forests.
The three protagonist system will return, with the new guys reflecting their GTA 5 counterparts. (poor dude, rich dude, phsycopathic redneck)
It will be set during the winter, this could work for both map options as Vice City is hot all year round, or with the Midwestern cities we'll see our first GTA set in a primarily snowy climate.
The custom GTA:O protagonist will have the option to either remain mute like in GTA 5 or an option to choose from multiple voices, reflecting their region of origin, although one could assume they're from North America or the Carribean.
There will be over 90 base game weapons. More will be added in content updates.
The minigun will not return as a personal weapon, although one could assume it's two replacements are the Rapid MG (MG3) and the Rail Gun (which is nerfed, it only disables vehicles now.) The minigun can still be seen on armed vehicles.
The homing launcher will not return, in its place is the Rapid Launcher (SMAW) that shoots missiles at extremely fast (4x the speed of the homing launcher in GTA 5) speeds. Homing rockets will still exist in military aircraft.
The guns will be far more lethal than their GTA 5 counterparts, with even the measly pistol being able to take down an unarmored guy with 6 shots to the body. Damage drop off no longer exists. This allows people with early game weapons to still be competitive with end-game players.
It is no longer possible to move around while aiming down the scope of a sniper rifle, this basically nerfs the sniper so it's no longer such a devastating weapon when one strafes around.
cannons mounted on aircraft don't shoot explosives anymore, but rather do Area Of Effect damage, this allows them to still be devastating while not game-breaking in free-mode.
Vehicles feel like something out of a dedicated driving game, like say the Forza Horizon series.
In this near future setting, Coil has gone from being a small company that makes niche cars and some military hardware to a massive multinational corporation, with massive stakes in the vehicle, arms, real estate, IT and finance industries. They play a somewhat relevant part in the game's storyline.
I can't predict the jump in terms of graphics, but one could assume it will jump the same gap as that between GTA 4 and GTA 5.
Rather than just a form of matchmaking searching, one could browse for gamemode lobbies while in freemode on their phone.