I have also thought about changing the monsters' defense to white shields, simply for rule aesthetics. I didn't understand why the rules treat defense differently for heroes and monsters for apparently no reason, and I always prefer the simpler solution of a unified handling. But after thinking about it for a good while I'm happy with the combat system as is. Here is my chain of thoughts
One challenge of dungeon crawl games and also adventure books like Lone Wolf or Fighting Fantasy is to make it possible that a single hero or a small group takes out about 20 monsters. The monsters should feel like a challenge. You would not be proud if you defeated 20 weak creatures. But then with fair rules, how is it possible that you have a good chance of defeating 20 equal foes?
In all these games and books they use monsters with equal or better stats than the heroes to make them look like a challenge, and then somehow hide the fact that the heroes are indeed much stronger. In HeroQuest the Orcs have the same fighting stats as the barbarian, and yet you defeat dozens of them!
The most common way to achieve this is to use body points. When playing Fighting Fantasy, have you ever wondered why your hero is such a tank compared to the monsters? In HeroQuest, why does the wizard have more body points than the gargoyle? Because else the monsters would win. The European version does a great job hiding this "cheat" by giving all monsters 1 body point. The body point system is not used for monsters, so the players are not hinted at their advantage.
Another way is to use an assymmetric combat system. In Lone Wolf you roll a D10 and look up the result on a table. The table is quite complicated, so at first glance you don't see that the Lone Wolf gets an advantage of about +6 on his combat strength compared to the enemies. You're proud to defeat the Gourgaz with combat strength 20. A fair combat table and a Gourgaz with combat strength 14 would not feel as heroic. In HeroQuest the monsters seem to have a fair chance of blocking when looking at their stats, but in fact the black shields come rarely. So the black shield defense is used to give the monsters good looking stats with a weak effect.
In HeroQuest the heroes have another advantage, this time without any unfair rules: They always have the initiative. So when working together, they are more effective. The "rolling dice for movement" rule adds a little risk to the initiative, which I think is a great mechanism. The advantage of the initiative comes from the fact that some monsters are defeated before they can act. To make this work, a hero's attack needs a good chance of taking out a monster. So the one body point + black shield defense rules boost the heroes' initiative advantage.
In HeroQuest all these rules work perfectly together, up to the point when the heroes start buying weapons. This is because of HeroQuest's
binomial distribution combat system. Let's call a result of 2 or more skulls a "critical hit", short crit. The 1 body point + black shield defense rules make it very unlikely for a monster to survive a crit, no matter its defense stats. In DnD like D20 systems the crit chances don't increase automatically with the hero's combat strength. In HeroQuest they do: 1 attack die has 0% chance of a crit, 2 AD have 25%, 3 AD have 50%, 4 AD even 69%. With heroes of 3/2/2/1 AD the most probable attack result is a single skull, and you feel the difference between a skeleton and a zombie in defense. With 4/4/4/3 AD the most probable attack result is a crit, making the monsters' defense stats pointless.
At this point Zargon feels the need to fix things. The first thing to do is to use more monsters per room. This makes the game longer in total, and also means you will always get punished for a rush because the monsters are so many. They did this in KK and RotWL: some rooms have 8+ monsters. But usually these rooms are meant to make the heroes flee, not to rush, so it is fine. But don't use this fix for regular rooms unless you like door fights!
The next thing to do is give the monsters body points, like the US edition. While this balances things a bit, it also directs the players' attention to monster body points, making it harder for them to ignore the fact that they have the unfair body points advantage over the monsters, which gives a less heroic feeling to defeating monsters.
The next thing to fix is the black shield defense as suggested here. With less equipped heroes this needs a fix to reduce the "half-life" of the monsters. Bareheaded Warrior found a good fix by counting the black shield as a skull, resulting in about the same survival chances for the monsters. But this fix leads to a higher damage output of the monsters, making rushes riskier for less equipped heroes: The success chances of the rush are about equal, but the damage on failure is higher. I have to say I really like HeroQuest's black shield trick of creating a hidden advantage for the heroes and wouldn't change it.
Most of the time we play a random mission from GS, KK or RotWL with no equipment. For GS we use the maps from the remake (more monsters), but EU stats (1BP). I have to say that all the maps have the right difficulty when the heroes don't have equipment (except for
RotWL M1). For younger players the maps from the European GS have the right difficulty.
So my suggestion is to step back and look at the root cause of the problems: The weapons from the armory. I think there is a narrow band of hero stats where the beauty of HeroQuest's combat system really shines. Permanently buffing the heroes' attack leads to games that are either too easy, or dragging because of stronger or more monsters. It also evens out the differences between the heroes, making them all feel the same, except for the poor wizard. And then there is the least heroic weapon: the crossbow, that allows you to fight from the open door like a wizard, but with unlimited ammo, turning the wizard into a wandering healing potion.
I guess many players would not like to do without the armory. Where is the feeling of progression without the armory? The reduced European armory with only 1 battle axe and crossbow would be a starting point. With 4/3/3/2 AD there is still challenge. The equipment costs and quest rewards should be designed in a way that this point is reached towards the end of the campaign. And I would expect new campaigns to be designed for a team of fresh heroes.