This guy designed a program to act as game master for running HQ without an Evil Wizard player. One of the things he did was make some kind of points system for the monsters so people could increase the difficulty if their heroes were too strong. He also created tougher varieties of the monsters we're familiar with, like a Skeleton King, or an Orc Captain. I can't remember them all. It's probably worth checking out.
http://heroquest.hjortskov.net/quest_ratings.htmEDIT: Got some more info.
I looked a little closer at the program and I thought I'd post some things I discovered. (At first the commas threw me, but I think they're meant to be decimal points.) Anyway, his formula for determining a monster's score is like this:
Score=((attack dice+defend dice)/2 + body points + mind points/3 + offset)/constant
The constant is 3 and the offset is 1. I'm not sure how he figured out the constant and offset but that's ok. Using that formula a Goblin's score is 1.3. The rest of the monsters work out like this:
Goblin 1.3
Skeleton 1.3
Zombie 1.5
Orc 1.7
Mummy 2.2
Fimir 2.3
Chaos Warrior 3.0
Gargoyle 3.3
So if a quest is too easy, you could replace Goblins with Skeletons, Skeletons with Zombies, and so forth, but that may not make sense with the atmosphere of the specific quest you're playing. So, you could replace Goblins with Orcs, and Orcs with Fimirs, or replace Skeletons with Zombies, and Zombies with Mummies, but then all of sudden you're not playing with the Goblin and Skeleton figures anymore.
So this guy came up with a neat upgrade system -- new monster using the same figure. I'll go back to my Goblin example. If a regular Goblin is too weak for your Heroes, upgrade him to a Goblin Guard: A-3, D-2, M-1, B-2, Movement-8. With those stats its score is 1.9, more of a challenge for your heroes and you get to use the same goblin miniature.
Here's another example using Skeletons. Their score is also 1.3, but they can be upgraded to Skeleton Guards, Skeleton Warriors, and then Skeleton Kings. A Skeleton Guard's stats are A-3, D-3, M-0, B-1, Movement-6, and a monster score of 1.6. Next is the Skeleton Warrior: A-3, D-2, M-0, B-2, Movement-6 and a score of 1.8. After that you could upgrade to a Skeleton King with these stats: A-3, D-4, M-0, B-2, Movement-6, and score-2.2.
I think it's a neat little points system (though I've never tried it out in an actual quest). Who knows if changing a score by a couple tenths of a point exponentially increases the difficulty. Anyway, I hope I helped a bit and didn't overwhelm you.