I would love to see a brush fire system put in place as to where you can see the size of a brushfire/forest fire etc on the map. The fire could also expand randomly on the map, and have the potential for spot fires. You could make use of the forest POI so it would only start in these types of areas where the poi is placed and wouldn't affect players in cities. It could expand every 10 or so minutes and each time it expands it would require 1 additional unit, and if you had a sufficient amount of units it would decrease in size every 10 minutes and require 1 unit less etc.
I know this would probably be impossible to implement but if it could be, i think it would add a great challenge to the game. I thought the idea was pretty cool to share anyway, i have posted a few examples below as i'm crap at explaining things.