Those sound very good. sounds like they will take advantage of the HL2 engine.
What if you had players running up a hill and you released a bunch of barrels all at one time and they came tumbeling down onto the players?
Does this have to do with multiplayer or what?
In single player I am sure they have some sort of spawn set for the ant lions to spawn when you throw one of those ant lion stench bombs. I am not quite sure about multiplayer.