The way my group is handling Keys is different than the standard way, so I'll try to suggest Keys that could be easily used for either. The Stealth Combat Key mentioned above, which I'm calling Key of the Assassin for now, just to have it named, could probably work if we give it a decent counter, maybe announcing yourself to your opponents before a battle, or attacking an opponent who's expecting you, I dunno.
I don't like Key fo the Guardian or Key of Fraternity. They're pretty much Identical, for one. For another, they have a limit on the number of times per adventure (a very squishy ammount of time) you can get the 1 XP reward. I don't think that Keys should restrict you in the ammount of XP they can give.
In our modified system, we'd just say Key of the Guardian, and the GM would give out XP based upon guarding the person the Key is tagged to from harm, allocating more XP for protecting them at great risk, and more for protecting them at great personal loss, for instance. Translating this to the 'old way' of doing it isn't hard for this Key, but for some others it can be. My GM also thinks that though there are specific triggers written up for each 'step' of a Key, they shouldn't the the only method for getting XP from that Key.
Some sample Keys, sans their counters, that are appropriate for the character I'm currently running (Much easier to think of when applied to a character, for some reason):
Key of the Influential: You seek to gain the ear of the politically powerful, and to sway them to your side once you have it. Gain XP whenever your actions or words impress a leader enough to gain their sympathy and trust, or whenever you use that trust to guide the leader to see things your way.
Key of the Reviled: You are a creature hated and feared by all who hear of you, and your friends are few and far between. Gain XP whenever you make an ally of someone who knows your true nature, or convince a group that you are the exception, not the rule.
Key of the Orphan: You've lost your family, and you seek to renew the ties with long lost kin or forge new bonds with those willing to call you brother. Gain XP whenever you forge a bond with someone that is deep enough to be called family, or whenever you discover a lost relative and rekindle your relationship with them.
Key of the Assassin: You're a calculating tactician who's favored means of combat is a well times ambush. Gain XP whenever you defeat opponents through use of guile and stealth, and gain more XP when you take down an opponent without giving them any chance to react.
Key of the Manipulator: You're a conniving bastard who turns foes against eachother and who no one who knows you well trusts. Gain XP when someone takes the fall for your actions because of a well placed lie, or when someone does your will without even realising it because of your manipulation.
Key of the Free: You're a free spirit who cannot stand to live within constraints. 'Unjust' laws and 'illegitimate' rulers hold no sway over you, and you feel those around you should be free of them as well. Gain XP when you help to liberate yourself or someone else from opression, or you take steps towards eliminating the source of oppression. (E.g. Slavery, a Tyrant, Prejudice.)
Other Keys for other characters:
Key of the Pacifist: Bloodshed is not your way, and you seek to curb the use of violence as a solution to problems. Gain XP when you stop or prevent a fight and establish peaceful means of coming to a solution, and gain more XP when you can convince two opposed groups to settle their differences nonviolently at risk to yourself.
- Kemrain the Keymaker.
