All shooters should have Arena mode
What an ingenious idea! A full-blown mode dedicated to single player goodness. If you want something where you want to take a break from whiners & griefers in multiplayer, where you want take out that aggression you've bottled up from your daily routines, or where you just want to play something without silly things like "story" or "plot" getting in your way, arena mode is it.
There should be a creed written somewhere: "All ye shooters, thou must hath single player modes in thy games."