Monster possibilities thanks to Giles' canon point include:

-Vampires
-Generic Demons (I do not have the Monster Manual on me, so I can only name a few off the top of my head)
-Human soldiers
-Multidimensional horrors the likes of which man is not meant to see

I would love to bring some stuff in from the Buffyverse. That is definitely the hope~.

If "boss" types are allowed, I'd like to bring in The Master or Twilight.