If Illithids are a mindcontrolling centered race and their products are brains, 2 options would fit pretty well - zombie decoys
or wisdom/clarity related.
Make a workshop were you can create Illithid brainmeals, which increase User's Mental stats/give boosts to Sciencedwarf, medical and alike. (You could make it like the Library, and a reaction you name "devour Illithid brain" so you can limit the Workshop to specific dwarves).
Further reactions could be instant happy thoughts (bringing from tantrum to ecstatic) or, if thats possible to mod, using a bunch of Illithid brains to reveal secret dwarfs/cult of the carp god (brains giving insight into secrets). I dont think it makes sense to be able to make an actual product out of brains, except for the alchemy/chemistry based extracts a user above mentioned.
Furthermore, the brain extract could make for a poison based weapon against enemys. No clue about the modding possibilities, but syndrome inducing rage, tantrumming, confusion, dizzyness and alike against foes. Either as syndrome Bolts or as breath attack (dwarf consuming bazillion brains becoming special caste, like the magic buildings).
Frost giants. I am only playing this mod since 1.7 or so, so i didnt see them being introduced and i am not entirely familiar with their physiology.. products that could be leftover might be ice essences, used for alchemy and weapon upgrading. For example you could combine frost giant products with your magma only metal - a metal that can only be melted by magma and an alloy cold enough to not even be meltable my magma. mostly flavor, "frostbrand weapons" or something.
New invaders. Your mix is pretty cool so far, only thing im missing is a spellcaster race (warlocks acting strange) and a highweapon race. I imagine orcs as being strong and brutal warriors, with raw powers instead of finesse. A race of agile and trained weapon users would be cool, their products could be used in the creation of superhigh tier weapons.
I really like the idea of special butchering products, would make breeding much more attractive (so far i only breed war animals and butcher everything else for the products pretty much).
For magic, i liked the suggestions Hugos suggestion. I have no clue about the modding possibilities, but having low level healing and debuffing/buffing would be great. Curing nauseous/unconsciousness from wounded soldiers early on, buffing speed, etc, inducing martial trance for high tech magic
Ps. Im sorry for my terrible spelling, im germand and didnt sleep last night - 1.8.1 keeping me awake