Yes there will be a sandbox gamemode and aswell a trainer in the game. That way you can also "cheat" and have fun in SP.
Further there will be for singleplayer:
- Campaign mode
- Sandbox mode
- Training area for like shooting
Multiplayer:
- Campaign mode -> player VS AI
- Maybe eventually PVP. Where the game runs on roleplay player civilian only. A mix of with AI is possible but mainly ambient and not active role.
Herogus3xD's post in What is the vision for the map layout? A large plot of land? Interconnected islands? was marked as the answer