Despite the fact that I am very partial to the LotFP specialist interpretation of the thief archetype, I just had another idea for how to do thieves after reading Matthew James Stanham’s excellent article on thieving abilities. He writes:
Moreover, and as Robert Fisher pointed out to me several years ago by ways of his writings on the subject, thief abilities are not just colourfully named skills, but frequently duplicate spell effects, such as silence, invisibility, knock, find traps, and spider climb. [Link to Robert Fisher’s page updated so that it works.]
Why not just give thieves the ability to cast those spells? They could get one every other level, or perhaps every level, either in a fixed order, or based on player choice. It would probably be better to create a fixed order, both to decrease character creation load and because invisibility at first level might be too powerful (on the other hand, is invisibility really more powerful than charm person or sleep?). Each spell could be used once per day, or maybe the total number of spells per day equal to level or level divided by 2. If you wanted to expand the list of thief spells, other candidates might include some of the illusionist spells and animal friendship (I’m thinking here of Alec from the Nightrunner series, and how he was able to pacify guard dogs by saying “peace, friend hound” in Elvish or something like that).
These abilities could either be conceptualized as an inherent ability to tap into magical power (this would result in a more supernatural thief class) or something closer to the spells of a hedge wizard, which would require the thief to keep a spell book and memorize spells in the same manner as the standard magic-user class. Idea: thief that tattoos his spell book all over his forearms. Further idea: magic-users consider thieves’ guilds to have stolen their secrets, and seek to punish magic-using thieves whenever they get the chance.
There is significant precedence for this approach, both in terms of rules for other classes and in terms of the inspirational literature. The paladin, for example, has the ability to cure disease, as the spell. Thieves gain the ability to cast spells from scrolls. And The Gray Mouser was a wizard’s apprentice before he was an adventuring rogue. Edge from Final Fantasy IV (great picture here), who was able to use ninja magic, also comes to mind.