I'd guess the opposites of the Signatories, largely.
So, Norse Gods with Valkyries and Einherjar- Grendelkin? Jotuns? Dishonored dead on a ship of fingernails?
Dragons?
Daoin Sidhe- Formorians
Svartalves- Light Elves
The Tylwyth being a first strike is interesting. Did Gwynn ap Nudd know something, or were his people considered dangerous?
What's the natural enemy of a vampire? A ghoul?
If the ghouls are Sumerian, I suppose some kind of Zoroastrian djinni?
I expect Gothic living weapons, given the Fomorians predilections. Also why they went for the Paranetters- the same conservation of mystical mass is probably at play.
Oh, and so much iron.