I've seen a few of these in other places, but here are some skills to spice up those monsters!
Goblin: Can move through squares occupied by Heroes.
Orc: Defends with 2 extra combat dice when adjacent to any door.
Fimir/Abomination: If the Fimir/Abomination does not move, they may attack any Hero they can see.
Skeleton: Can attack diagonally.
Zombie: Attacks with one extra combat die while adjacent to another undead monster (skeleton, zombie, or mummy)
Mummy: Can move through walls, but cannot end their move in a shaded area on the Quest Map.
Chaos/Dread Warrior: If the Chaos/Dread Warrior inflicts any damage, they may move the victim one square.
Gargoyle: Once per Quest, the Gargoyle may cast the Chaos/Dread spell Ball of Flame on any Hero they can see.
I had a few goals in mind with these abilities:
1) Add more variety to combat without slowing things down
2) Fit the "theme" of each monster, without adding new monsters
3) Discourage Heroes from using the ol' door-blocking strat!
Zargon may use all these abilities from the start for more experienced Heroes, or introduce them one by one as Heroes progress.
It can be fun to read special flavor text as the monsters make use of each skill, ie: "These goblins are clad in dark cloaks and wear soft boots. They move with exceptional cunning, and can pass through spaces occupied by Heroes!"
What do you think, and how do you like to beef up your own monsters?