We just play custom maps in Alien Swarm

http://store.steampowered.com/app/563560/

Alien Swarm: Reactive Drop extends Alien Swarm, bringing more of everything: maps, aliens, game modes, guns... And most importantly Steam Workshop support.

• Tactical co-op for up to 8 players with a top-down perspective
  
• Steam Workshop support for community maps and challenges
  
• New co-operative campaigns
  
• Challenges: Modifications of the game, just like Mutations in Left 4 Dead 2
  
• PvP: Deathmatch, Gun Game, Instagib, and Team Deathmatch
  
• Singleplayer: Play with improved bots on all our official maps
  
• New Aliens: HL2 antlion guards and more
  
• New Weapons: Desert Eagle, Devastator, and Combat Rifle, with more to come
  
• Over 100 Steam achievements
  
• Leaderboards: compete with your friends for the fastest mission completion
  
• Improved spectating: see hacking minigames and mouse movements in real time