[Ugh, today is the day of edits...]
I'm thinking about making a dirty bomb. It would be nothing more than a small bomb with radioactive material which - of course - contaminates the area around the explosion. The only problems: It would have a very big radius and would make the whole area very unhealty for a long time (like... 10 minutes or longer?). Another idea would be that it sets free toxic gases over a big area...
Just another random idea...
I would prefer gas. Maybe a bit like the current C4 for the bomb which you can time, but if in a hurry you right click to use a detonater.
EDIT: Also, maybe make it a bit like the teargas grenade, except you slow down, and everything starts glowing green or something.