I think the first question you need to ask is what kind of acid are we talking about?
For example, if it’s “stomach acid” (dilute Hydrocloric acid), it’s going to irritate your eyes and sinuses if it gets on/near them, but that’s about it. This would act roughly like boomer bile plus some pain.
Concentrated HCl OTOH is going to do severe damage to sensitive tissues like mucous membranes, eyes, and sinuses, and cause immediate burns on exposed skin. In-game you’d be looking at painful but almost certainly non-lethal burns on exposed skin, and significantly more painful but still not lethal burns on more delicate tissue, possibly lethal if the player managed to inhale significant quantities of aerosolized or fuming HCl. See http://www3.epa.gov/airtoxics/hlthef/hydrochl.html for some details, the main takeaway I see is “moderate to high acute toxicity from inhalation and moderate acute toxicity from oral exposure”. If I’m reading this right, that’s “significant chance of death if subject inhales HCl at a concentration of 2,000mg/m^3, or swallows ~2,000mg/kg of body weight”. Those seems like very heavy exposures to me, but feel free to correct me if I’m not reading those correctly.
Even at those concentrations, HCl is still going to do next to nothing to sizeable physical objects, you can look up some youtube videos of various things being put into HCl, for example a soda can, which looks like it took hours to weaken enough to make a seam burst from the carbonation pressure, not exactly exceptionally menacing. Another video I ran across had a chicken leg suspended in HCl for 18 hours, at the end of which the immersed portion of the leg had been dissolved, but we’re talking 18 hours of exposure with the volume of acid being higher than the volume of the dissolved flesh.
I think it’d be pretty cool to overhaul spitters to where they attack with an acid spray, perhaps targeting the face, with the initial batch of spitters producing regular gastric acid, and successive evolutions of them producing more concentrated HCl.
Now if we wanted to be really mean, we’d give advanced spitters the ability to produce and launch a Chlorine-producing compound, that’s significantly more toxic than just HCI, and even relatively small puddles of it would produce dangerous clouds of Chlorine, not to mention a direct hit on the player causing the player to become a source of Chlorine clouds.
I think this sort of thing would be a far more interesting way to take “acid” rather than trying to keep it as an abstract game entity with no reference to reality. For higher levels of danger, there are other chemical compounds that are incredibly lethal in very small quantities, such as igniting on contact with air (fire-breathing anyone?), reacting with atmospheric water to produce large clouds of concentrated sulphuric acid (reportedly dissolves heavy rubber gloves in a matter of seconds!) or acids that are eagerly absorbed through the skin and cause heavy tissue damage with even slight exposure.
