Hey, if you want some more alien swarm maps to spice things up check out these
http://swarmarmory.com/
additionally creating a shortcut with this in the target allows singleplayer on all maps so you can play them when no servers are hosting them

"D:\Program Files (x86)\Steam\steamapps\common\alien swarm\swarm.exe" +asw_show_all_singleplayer_maps 1

Make sure the above is all one line with a space after the closing " and the +