Is the field of play going to be small or medium sized for speedball types of games, or are you going to have to hike for several minutes before you even see the enemy team?
If it's big , I'd say get as much natural cover as possible, and cover bare areas with artificial cover (plywood barricades, etc) then cover that with shrubs / trees etc. Guard towers or sniper towers would stand out as a big man made structure in the middle of a field / forest, so you will have to even it out.