Perhaps the defining feature of the Thief (Rogue/Ranger/Whip-Wielding Archaeologist) is not a particular skill set, but the sheer amount of luck these characters seem to have on tap. I present the Luck-Based Thief:
Armor: Light Armor (+3 AC) only
Weapons: Any Light One-Handed Melee, any Light Ranged
Starting Saving Throw: 13
XP per Level: Same as Cleric
Prime Attribute: Dexterity
Luck: A Thief gains a number of luck rerolls per day equal to ½ his level plus his DEX modifier. Any die roll may be rolled twice, taking the best result. The use of this ability may be declared shortly after the initial die roll.
I originally envisioned this class as a Divine Thief, who would pray to the gods for an hour every day to receive his luck. It is easy to see how it could be modified to fit any number of archetypes.
I like this approach because it encourages the character to take risks, to rely on his luck and his wits, which is for me a much more satisfying play style for the Thief. Also, it means there is a possibility that his luck could run out in a tight spot, and one can see the sweat beading on a player’s forehead after the first four re-rolls have failed him in a life-or-death situation…