Far Cry 5 is the perfect co-op sandbox game
I have started playing Far Cry 5 with my wife since last 10 days and this is the greatest coop game we have ever played. The level of freedom to go anywhere and still run into interesting encounters/scenarios/missions is just great. This is kind of a dream co-op game especially for my wife who doesn't like Linear games ("Don't tell me what to do"). She just loves going in guns blazing while I stay back and try to snipe or attack from the sidelines.
The amount of surprises it throws at you is also great. Great Companions (animals or human), so many tools and vehicles(How cool is Driving the helicopter!) and seeing the whole Resistance rising while you go about liberating hostages and holdouts is just Chefs kiss.