Clouds: When you are too high (with jetpack or selfboost,...), there will appear a lot of clouds who make it difficult to see.
Fog: You cant see anything, except a little area around the player. The sight of characters will become bad too.
Something like this --> http://i.imgur.com/BGOHfV9.png
Smoke: Clouds of smoke caused by vehicles/machines who disable the snipers lazer pointer.
I would like to see such things in PB2.5