I can make a tank (or a butterfly), then blow it up with lightning? I think DayZ just got topped in the pantheon of "reasons to own ArmA". Check out this short trailer that shows off some of the features.
ArmA 3: Zeus is coming soon as a free DLC for the game, and it looks incredible. It lets one player become Zeus and create new objects, events, and objectives for other players to experience. It's basically Sleep Is Death with guns, and what more could you want?
Update: This article originally referred to Zeus as a mod, but it's an official DLC by the game's developers, Bohemia Interactive.