I explicitly mentioned that the module would only apply this to "debuff" potions to avoid this issue along with one other. However, it would also be interesting to try it without a debuff restriction because it could add more variables to the meta. Defenders could heal themselves for less potions at the cost of also possibly healing the attackers, and poison clouds could be applied inside the core room, etc. There is a possible issue with golems, where a regen lingering cloud could regen a defending golem or a lingering poison to kill an enemy golem. However, I believe the golems don't really use armor, they just have a lot of health, so the effect may not be significant.
Also, there's no reason that the module also couldn't change the duration of the lingering cloud or its potency. Using the default lingering potion of a debuff would probably be a bit weak, so increasing the lifetime of the area_effect_cloud and possibly the duration of the potion effect it applies seems reasonable.
The lingering cloud module (however it would work) was also one I thought of recently as well. Just something to shake up places to both go OR avoid on a battlefield would add some fun to otherwise boring fights. Especially if these fights were somewhere with purely flat terrain.
I think my thought was that the module actually "flung" projectiles in an area around it, and wherever they landed there would be a lingering potion cloud that does something positive to defenders, negative to attackers, or even both.