I was dreaming about this last night about..... Flame throwers in spiral knights, including: Ice thrower, plaque thrower, flame thrower, haze thrower etc.
So I thought this could be a good idea :P
The throwers spits out a wave of fog(depends on if it is fire, freeze, poison or something else) Just like zombie series's fog attack.
Plus the Brandishes & Alchemers fill that niche already (though admittedly there's no Poison versions of the Brandish/Alchemer, which is kind of a pity.)
Read some repilies and had some delicious searching bars :3. Vaporizers does have a large radius, but people need to charge the bomb before putting it down but the thrower weapons doesn't need charges .(By the way, throwers doesn't have charge attacks) So the throwers can be activate at any time without any charge time.
there are vaporizer bombs that do exactly what you described, over a significantly large radius