Some more tweaks for Donjon. When my fellow players like these as well (and I'm pretty sure, they will), there's hardly anything left from the original rules. ;)

Anyway, I really like the variant classes for D&D. The concept is so elegant: You give an ability away, to get another one. Almost philosophical. So here are three options for Skills. Of course in Donjon a skill can look almost any way you want, but it cannot work any way you like. In fact the rules are pretty strict in certain areas.

Armor-piercing skills
Sometimes it makes sense that a skill ignores armor. But wouldn't it be unfair, if just any skill could do that? Probably. So make the skill officially Armor-Piercing. Targets of Amor-piercing Skills do not profit from armor, but the attacker can never profit from his or her weapon either. Even if the skill is called Backstab With A Knife or something. Sounds fair, doesn't it?

Quick skills
The normal rule is, that any skill roll requires an action. But shouldn't your berserk be able to use his Intimidate skill while he is beating up people? After all that seems pretty intimidating. Of course, he should. So make Intimidate a Quick Skill. Quick Skills do not require an action, but can only be used once per character level during each scene. This option works fine for special attacks like Fight With Second Weapon or Strike With Tail, too.

Unusual skills
Normally certain types of actions require certain attributes. But you're a Dashing Swordsman and want hurt people with your Sociality instead of your Virility. We understand your needs perfectly. You want to use your highest attribute for all the good things. So help yourself to an Unusual Skill and link it to your favorite attribute. As a side effect Unusual Skills cannot exceed your character level (instead of character level +3) and you can only have one of them.

What do you think? Something too weak or too strong?

Another thing that is pretty much ignored in my play are the saves. Skill dice are just too precious to spend them on saves. Obviously my players feel like that, and I'm not very inclined to buy them for monsters, either. So I'd say that both saves start at one. During character generation the player chooses a good and a bad save for the class. The good saves gets +1 at every level divisible by two. The bad one at any level divisible by +3.

Skills can be used to add to the saving roll normally.

Unusual Skills
As I understand it this is really just creating a rule where no rule yet exists. The rules only state that an "applicable" skill and stat are used. It doesn't say, however, just exactly what qualifies as applicable. The example given to me was that of exploring the cave. You can roll Discernment to check if you can see the hidden exit. You can roll Cerebrality and say that the PC recognizes all of the signs of the ancient DooDaa tribe in the construction of this cave, and a hidden exit should be right about...here. You could roll Adroitness and say that the PC... Well you get the idea. The point I'm making is that any skill can be used if and only if the player can describe it such that it makes sense (your "Dashing Swordsman" for example would simply be attacking vs. their sociality skill instead of wherewithal).

Quick Skills
Really you've described two skills that are about the same (and I wouldn't allow my players to take both, if they apply to the same situation the way you've described). Now, he could take stones of success against the opponents sociality skill, and then try to beat them in such a roll. Or he could state that his barbarian is intimidating the shopkeeper by beating the living daylights out of him. Or, he is intimidating the shopkeeper by breaking his customers in two. Either way, not all successful rolls need to be put into doing damage. Some of them can be applied to stating facts about the environment, or those in it.

Armor-Piercing Skills
This one sounds like it is just stretching. But I just can't figure out how to explain why.

Oh, you're not quite right about the use of attributes. Outside of combat you can use almost any attribute all the time, of course. But when you're done with finding secret corridors and get to swords and fireballs, the book is pretty clear about the responsible attributes.

Also I think, you missed my intention: Offering strategic options.

The way I have always thought of abilities is that every time a player comes up with a new ability, the GM and player jointly write up a new bit of rules text that describes exactly how that ability works.  In some cases, this rules text can almost be left unwritten, because the ability's interaction is very straightforward, as in:

"Awesome Strength"
This character can deal incredibly powerful blows.  Add "Awesome Strength" to melee damage tests.

But sometimes the rules text associated with an ability can be fairly complex, as in the description of Spraycaps from the rule book.  If you work from the perspective that each ability is really just a new little rule on how the player can use the character to interact with the world, then the kind of higher level rules changes you are describing don't seem to me to be necessary.

For example, here are some abilities and one weapon:

"Find the chinks"
The character is adept at getting through people's armor.  When using a dagger or other pointy weapon against an armored target, add "Find the Chinks" to the damage test instead of the weapon's value, but the opponent does not get to add their armor to the damage test either.

Knight's Backup Dagger
Worth 6
The Knight's Backup Dagger is a long, tapered, very hard dagger that can be used as a medium (1 die) weapon, as normal.  It also has the ability "Pierce Armor 5", which can be used instead of other damage adding abilities when using the dagger, but only against armored targets.

"Brutal Intimidation"
This character is adept at beating people up to get them to do what he wants.  Roll Virility+Brutal Intimidation versus either Werewithal+an ability to endure the beating or Discernment+an ability to ignore the mental pressure of the intimidation and to not give the intimidating character what he wants.  Success can be used for facts ("You are cowering in fear") or in an immediate damage test.  In any damage test that results, non-bladed weapons such as baseball bats, whips, etc can be used, and the opponent also gets their armor in the test as well.  The Damage Test damages If the opponent wins the test, they can also state facts or do an immediate damage test.

"Dashing Swordsman"
This character relies more on style and grace in combat than physical strength.  When using a stylish weapon (fancy rapier) and dressed in a stylish manner (very fancy clothes), this character uses Dashing Swordsman in damage tests with the stylish weapon, and uses Sociality instead of Virility.

It would seem to me that the above pretty much do what you are looking for, without any new high-level rules changes.  The key is just to define the circumstances in which the ability's use is appropriate and be creative. 

"Brutal Intimidation" and "Dashing Swordsman" are fundamentally the exact same thing, and admittedly stretch things the farthest; in both when using the ability you are using an alternate attribute than the one that would normally be used.  With "Brutal Intim." you are essentially doing a normal attack, but using Virility instead of adroitness on the attacking side, and Werewithal or Discernment instead of Adroitness on the defending side.  With "Dashing" you are swapping Sociality for Virility in certain damage tests. 

As a player, it may SEEM like you want to always use your best attribute.  But remember that while it is very hard to kill a character, it is easy to make the virtually worthless through attribute damage, and putting all your abilities into one attribute basket just makes this worse, as in:

GM: So, lets see, the Ogre attacks Henri the Dashing Swordsman and hits.  Hmmm, now, Henri attacks using sociality, right?
Player: Yep!
GM: and he does damage with Sociality as well, right?
Player: You got it!
GM: and he even resists damage using that incredible Sociality, right?
Player: *starting to get worried* Umm, yes?
GM: Cool, then with these five damage successes, I will use one for a fact, your face has been smashed, and then put the other four into Sociality damage.  Wow, with a normal character that would have taken three times as much damage to get the same effect, thanks!
Player: ACK!

An ability like "Brutal Intimidation" relies on a communal understanding of exactly what can be achieved in terms of forcing character's to take actions through facts.  It's essentially the same issue as you get from any kind of Mind Control magic.  Is "You will do what I say" enough of a fact, or do more have to be used to actually force particular actions?  How long does "You will do what I say" last, if it is enough of a fact?  The rules provide little or no guidance on that issue.

Thanks Hans. You worded it much better than I could have. You also included something that I couldn't find the words for. That the rules governing an abilities use are defined at character creation, no matter what the ability is or does.

Thanks Dragon.

It just occurred to me that there should be one other change to the write up for "Dashing Swordsman".  It should say that you do NOT have to spend a fact to damage Sociality, but you DO to hit Flesh Wounds.  That reflects the fact that its easier to make a person look stupid with "Dashing Swordsman" than it is to actually hurt them. 

I also note that the "Brutal Intimidation" description has a mistake in the last sentence.  It should really be two sentences that read "The Damage Test from Brutal Intimidation damages an opponent's Discernment without using a fact, but you must spend a fact ('I break your arm') to do Flesh Wound damage.  If the opponent wins the test, they can also state facts or do an immediate damage test themselves." 

"Find the chinks"
The character is adept at getting through people's armor.  When using a dagger or other pointy weapon against an armored target, add "Find the Chinks" to the damage test instead of the weapon's value, but the opponent does not get to add their armor to the damage test either.

Stefan wrote:
The problem is that there is no rule that allows for such an ability. And I as the GM cannot make it up because my job is to use every available rule to kill the characters.

If I could made up new options on the fly that would either be cheating or outright nonsense like a referee trying to score goals.