At risk for necromancy warnings, I know, but this is a really interesting subject. The Stirges were particularly well written, and I think my favorite part of this grand experiment is going to be the smaller, weaker horde of monsters that are CR 0-4 that need some love. I'm thinking of magmins, off the top of my head. The only one i've ever used as a DM was a mind flayer's slave, used to torture people. I'm not sure how I could justify using them for much else; they're essentially fire elemental, not-quite-evil gnomes.