Make sure to post a video of it working when it's complete.
As far as ideas go, you covered most ones I could think of. Here are some bad ones I thought of:
A long hallway with floor grates above magma, so only the toughest make it through without dying of heat. To make it more interesting, chain a GCS to a location where he covers those crossing in webbing, forcing them to further endure the heat.
A long 1 tile wide bridge, on one end is a pit 2-3 z levels below filled with carp and 4/7 water. On the other side is a wall with wall grates. Upon hitting a pressure plate, a wall of water comes through the wall grates in different portions of the bridge; causing anything/anyone crossing to be hurled into a carpy death pit with broken legs.
A beer-bombing chamber. You have a separate room with a pressure plate set to activate in 6/7 water, the room is 10x10 and all tiles are full of 7/7 water except 1. The plate triggers BOTH doors to the bombing chamber. You use the ignition method to create a bin full of burnging lignite, and have a few bins drop in randomly or somehow automatically from an above hatch. A level above that hatch is a 1x1 alcohol stockpile. The alcohol drops down and explodes into the room. Now, scale this up and you have a random opening and closing exploding chamber.
Use a balcony full of Goblin Sharpshooters that immigrants run by.
A hallway full of cages that contain deadly creatures with leg wounds. Create the wounds however you want (Dropping them and hope the right parts break) and let the immigrants run away from beasts that are slightly slower then themselves. The only catch, is initially they need to run through them.
Have the area that they initially gather into 1 large locked room. Then, release an amount of unarmed goblins 1/5 the size of the immigrant wave. Let the survivors run the gauntlet.
The room before the end of the chamber should be a dwarf on dwarf death arena. You cage up a number of berserk dwarfs equal to the number of surviving immigrants, and have them fight each other to the death!
The final room should be an insanity chamber. Dwarfs who enter are provided with the minimum number of basic amenities to keep them alive (IE: Murky water, occasional food drops from hatches, rotting body drops) and the entire room should be covered in cage traps. Over time dwarfs will all go insane, and can be used to battle immigrants. For bonus points, when they initially enter the room have a weapons stockpile for them. Arm them with no armor; and wooden hammer/other violent and humorous weapons. You can also arm a few with crossbow and bolts/quiver, and when they go berserk release them into the gauntlet on balcony positions overlooking the gauntlet for them to fire on helpless immigrants.