I like the idea of the gas expanding, but perhaps getting weaker (ie you can spend more time in it) as it does so, down to a threshold at which point it disipates. That said, keeping track of how long you've spent in the gas, plus displaying different densities of the gas seems like a daunting task.
Also, what if the gas affects monsters, but perhaps in different ways. For example:
1. roaches, roach queens and spiders might be immune.
2. Goblins, golems, aumtlichs and other humanoids are susceptible to it (possibly immediately rather than gradually... keeping track of which monsters have more time allowed is daunting).
3. Wraithwings (and flying fegundos) might push the gas away from them as they beat their wings (possibly leading to increases in density in other areas). Imagine having to follow a group of WW through a moving bubble of breathable air within the gas cloud.
4. Exploding Fegundos and bombs might ignite the gas.
5. Tar-stuff babies might die if they are alone, but if caught in the gas in a homogeneous 2x2 or bigger group, might fuse into tar-stuff.
6. Snakes might swallow gas at their heads, and, err, re-emit gas periodically from their tails.
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