Vox: Increase penalties for being maskless, like upping the toxin damage they receive. Because currently you can take off the breath mask and sit for 7 minutes (give or take) before the damage reaches 50 points, where it starts being dangerous. At the moment, being a maskless Vox isn't as life-or-death as people think it is, especially when you can down some vendor mugwort tea and be right as rain in under a minute.
Slime: They're made of water, one could give them a negative reaction to the Drying Agent chemical.
Grey: Give them other minor psychic powers to choose from at the character creation menu, so they could pick the Remote-Talk power to have or something else. Like the ability to summon a tiny flame into their selected hand on activation (like how some Wizard Touch spells work) that would give them all the benefits of holding a no-drop always-alight match; one that couldn't do anything a normal match or lighter couldn't.
Another potential ability could be one where a Grey could empathically take all/half of the pain/stamina damage of a target, at the cost of needing line-of-sight or a grab. That could also come with appropriate downsides, like getting stunned if the target is, or taking heavy brain damage should they die.