I would definitely say a semi-urban environment, where there is a mix of close quarters and field gameplay. I love the idea of the 24-hour continuous game where the players go on patrols and such while they wait for an attack at any time; however, there needs to be some sort of rough schedule and multiple strikes against each team's base. Otherwise, the game would end up with multiple players sitting around not doing anything because they became bored of not shooting at people continuously, possibly leaving before the op is over.
