How about each round having one or two related missions that lasts throughout the map? Like; "You're getting transported to cs_italy to deal with a headcrab infestation" in the map change screen, and when you're in the game you choose what corp to work for. Then There could be 5 headcrab missions, and when they're all dead, there's a mission where you have to kill something big, or half of the players turn into zombies or something. That way you could get a lot of variation in the gameplay. Let's say there's 6 of these 'gametypes', and you've got 20 maps in the mapcycle. That's 120 diffrent situations that could last up to 10 minutes each, depending on the amount of time you put into each gametype.