If you ask LPJ, he'll tell you "We need more monsters!". But that's about all the inspiration he'll give you. Or he'll say "it has to be scary". That's helpful.
When the time came to put together the NeoExodus campaign book, I thought to myself: I need monsters that are interesting, iconic and can help a GM set the mood for NeoExodus. By that time, a couple of monster books were already available: the First Ones, the scythians, the calibans, the harvesters of sorrow, to name but a few. I started in those book and took a few monsters: the low-level scythian warrior appeared again, I copied the caliban from the caliban with some (minor) updates to it, based on material that was not available in the original book. Adding a fully stated Exodite as a First One was a must. So I built an Exodite inquisitor. Bad mojo. Finally, I added the Quickslavers, as they were in the original book and that they really added to the dark elements of NeoExodus. Those things are evil.
Going back to old NeoExodus books, I found a monster forgotten: the draco. The draco was a race LPJ designed in the pre-4e days aka "the preview days". It was to bring dragonborn to NeoExodus. The idea was good, but I found them a little lackluster as a race (in 3.5). We already had enough races for the book and their original book had some decent background for them making them isolated from the rest of the world. So I decided to remake them, but as a "monster".
Monsters from a previous adventure are the necryos. These 3.5 cold-base monsters were given a make-over and streamlined abilities, making them pretty scary against unprepared adventurers, giving the GM a nice advantage on the PCs.
With those monsters done and updated, I thought "what other creatures do I need?" The Protectorate Golem - or Ironforged - immediately came to mind. They do appear in at least two of our yet unreleased adventures... So I built a few sample ones: a basic ironforge, the military ironforged, and the ironforged goliath, a final, smaller and more common one was created to round them out. The now-infamous mobile cannons (anyone who faced them know how bad they are). Protectorate golems are all partially intelligent (Int 3) and have the profession soldier skill. Still they are dumb as a rock, hence the constant need for Protectorate officers and artillerists to command them.
The other monsters added to the book were either, for flavor or things I like lot. The Arcaneslime, the caliban hound, the cynean hunter, the ember, the melted flesh ooze, the needlewing, and the razorfiend complete the monsters.
The result is that in the Campaign book, you have monsters that span the gammut of CRs: from 1/3 (caliban warrior) up to CR14 (melted flesh ooze).
If you have comments about it, contact me, I'd love to pick your brain about it!