So, watched the big long gameplay demo.
It's basically MGSV with a much bigger open world sandbox, more robust traversal mechanics, and some survival elements.
Core gameplay loop is: get a mission, pick a loadout, work your way to your destination, encounter and scout stuff along the way (enemy bases to stealth through, minibosses to fight, side quests to complete, things to scavenge), manage your gear and health, and ultimately reach your destination.
In the process you'll encounter signs, paths, tools, packages, ladders, bridges, ropes, vehicles, etc. left behind by other players, and you can do the same. There's a like system built into the multiplayer, which allows you to upvote the things that other players have placed in the world.
The overarching flow of the game is traveling from the East Coast to the West Coast of America, connecting cities along the way to the Chiral Network, which is the network that links all players together. Areas that you don't have activated yet won't benefit from the multiplayer collaboration.