I've been using this for a little while now, and I have a few ideas to help improve it.

1. MAKE THE SPAWN MENU RESIZABLE
This might be a bug, but the spawn menu currently can't be resized. I think that should be fixed, since the default size is really tiny and hard to work with, especially with longer lists of NPCs. If you only take one suggestion per post, PLEASE let it be this one.

2. Speed multiplier
You can currently change the size and attack strength of all NPCs. That's cool. But this could be expanded on a lot to allow for even more options when designing a new foe. The most obvious choice would be to change the movement speed of an NPC, making them faster or slower at your discretion. You could have a giant headcrab that moves slower than its comrades or a fast zombie that's even faster than normal.

3. Allow NPCs to summon other NPCs
Zombies spawn headcrabs on death, and poison zombies even throw a few of their headcrabs at you. If you could make an NPC summon something else, either on death or as an attack, you could potentially create full-blown bosses that are more than tougher versions of existing enemies.

4. NPC Coloring
A much smaller change I had in mind was the ability to have NPCs spawn with a particular coloring, as if someone had used the tool on them. This could be used to mark a particular NPC as special without messing with its size. It could be used in themed invasions, having every NPC be a particular color to show that they're part of the same group.

Hope you see these, let me know what you think!