You know, my initial thought was that the 4 subclasses per class would line up with the 4 class groupings (and maybe it still will). But this opens the door for multi-class via subclass. IE a subclass version of each class that is built to function with any class. So you could be a Cleric/Warlock or Warlock/Barbarian.
Perquisites, proficiency in diplomacy, at least 1 knowledge skill.
3: advantage on knowledge skills.
Gain proficiency in Harper code, able to read and leave messages for other Harpers.
6: add 1d8 radiant damage. Do this a number of times equal to your proficiency bonus.
10: you are always under the effects of detect evil and good.
14: add 1d4 radiant damage to creatures you detect with good and evil
Perquisites: proficiency in thieves tools and stealth.
3: you gain Expertise in thieves tools and stealth.
you gain advantage against traps
6: you can cast invisibly, find traps, or knock a number of times equal you your proficiency level. Knock does not make a noise. If you have spell slots you can also cast it with them.
10: you are always under the effects of find traps.
14: you have reached the pinnacle of a silver key's prowess, this potent ability allows you to make use of any magic door or portal you come across as though you had the specific ability to do so. In particular, you can bypass arcane lock spells as though you yourself had cast them, ignore glyphs of warding as though you knew the password (even glyphs that have no password), and bypass the magic of a hold portal as though you could cast knock at will.
Your power even extends to portals created or maintained by magic. You can follow another character through a phase door, and can enter a Mordenkainen's magnificent mansion, a dragonmark demesne, and similar structures regardless of whether the caster designated you as one of the creatures able to enter.