This is probably an incredibly stupid suggestion which I haven't thought through properly in some way, so I'll just throw this out there.
Trip mines: A bomb which has a small, rectangular blast radius. When placed, it takes time to arm, then, when armed and a monster walks over it, it does damage to them.
Now for the important bit: BALANCE.
So far, I have a few ideas:
1. A limited number of mines. This would mean that people would not be able to spam them like crazy. Mines are resupplied at an Arsenal Station or at the beginning of a level.
2. A 'cooldown' on mines - you have to wait (example: 30 seconds) after placing one in order to place another.
3. A massive damage decrease, to below the damage of other bombs.
4. (Courtesy of Sourix) A limit on the number of mines that can be placed at once - for example, you can only put down 3 at once, but if one is destroyed/you remove one using another attack with the bomb (Confused? Attack near the bomb while equipping the bomb to remove it), you can place another. Placing one over the limit will destroy the earliest laid mine.
Then, of course, balance in Lockdown.
Here are a few other guidelines for LD only:
1. Mines can be removed by being hit with any weapon - gun, sword, or bomb. Gun attacks will explode against it automatically, not go over it. AA applies to mines.
2. Mines will be filled when a player enters the spawn zone behind the mist barriers or dies, BUT all previously laid mines will be destroyed.
OK, so I'll throw that in. Please do not be kind and leave out any flaws you have spotted - constructive criticism appreciated - and I am planing to develop this into a fully working idea, not just a concept.
EDIT #1: Please bear in mind:
1. These are not offensive bombs; they are designed for defensive purposes, ie. covering your fleeing rear-end.
2. Only ONE of the options to balance the bomb in the Clockworks will be applied.
~Raisinfist.
small radius? limited numbers? cooldowns? decreased damage?
There's no reason to use them over regular bombs.