Ozai may have potentially been a threat, given time to lead from the rear, but if you're looking for the way to hurt him most, Aang pulled it off - he spiritually gelded Ozai. Heck, at that point, my scenario of Ozai rebuilding his power is a slim possiblity at best, as Korra described. That's something that's endemic to the franchise going back to G1. Really, the Decepticons could have just backed up to the Asteroid Belt, strapped engines to a few asteroids, and just lobbed giant meteors at Earth until "Rocks Fall, Everyone Dies" became the new status quo. If they didn't want to go that far, they could engineer plagues, hand WMDs to the various bad actors on the world stage, anything. Instead, they sat on their thumbs Darkness Rising Starscream really was a better leader - he knew how to play his cards close to the vest and preserve profitable, useful operations. If anything, he's the only treacherous Starscream to have a point - this version of Megatron was a jackass.