How about you make it so it gets his mist ball back when it kills a human? I imagine the mist bomb will be that mist cloud spreading effect that happens when you kill it in the icy addon map for GF3, right? Just do it so it only lasts for a moment (otherwise people would set up us the bomb on inescapable places). Should make for some interesting moments.
EDIT: And an achievement named "Mist Bomber!" or something for killing 2 or more humans with a Mist Bomb.
Aside from that, I was playing AEoD and used the grey looking shot of the Experimental Cannon. It looks decent enough to be an attack too, though I'd rather stick with the Mist Bomb since it brings something new to the Ghoul team.