Pressurized toxin cutters. It slices and it poisons at the same time! Perfect for use against unarmored biological targets!
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No this amuses me. Someone else come up with something to weaponize! Something icky and biological!
Get Moriarty on the task of creating a method to extract microbial genetics. Microbes have some ungodly toxins and other chemicals of use. As for the sort of thing you are probably looking for, a blend of toxins, acids and adhesives can ruin almost anything, including metals. Make the mix conductive, and you gave a handy electronics ruining goo. Replace the toxins with flamables and you have acidic napalm. The best part is that each form is just one chemical different, so it would be easy to have the Zerg that use these blends to swap to the most effective version. Electronics frying for anti-mechanical, napalm for anti-bio and low heat-tolerance armor and pure acids for what doesn't get wrecked by the other two.
There's also silicon-carbon replacements, where you swap carbon for silicon, making less stable compounds that can degrade into Quartz. It would give the swarm a way to make use of silicon and make it possible to make electrical Zerg due to the electrical properties of Quartz. Lower stability also has the aplication of having certain high energy compounds that would be impractical to ignite the carbon equivalent of. And you can always use the silicon compound as the igniter for the carbon compound. Interestingly, the chemistry makes it so that the same set of enzymes can make both the carbon compound and the silicon compound, at least in some cases. The biggest advantage is that your swarm could actually digest sand and computers for nutrients. And the wonderful world of silicon compounds would be opened to you.
Also, Changelings-as-larvae. Changelings can alter their cellular structure freely, like a less-bullshit Ditto. Add in the larvae's form of metamorphosis and you can accelerate production like crazy by having them permanently transforming into a wanted unit. Alter the genetics of the Changeling to include some useful traits and you can get one with a "toolbox". Acids for attacks, adhesives for trapping and fixing, explosives for sabatoge and high durability materials for conventional building. Those could turn into fragmentation bombs, acid bombs, glue traps and walls.
An interesting thing you could do with larvae is make use of the canceled Mophlisk larva varient as a production accelerator or make larvae into a combat unit for a dirt-cheap deposable unit... which might well be able to take anti-tank weapons.