Thief characters have a range of skills available to them, but they could easily be swapped out for others, allowing the thief to customize his play experience, or the specific type of thief.
Whether this is selected by the player or the DM, skills are swapped out on a 1 for 1 basis. The new skill will follow the chances and progression of the previous.
Generally, replacement skills are not subject to racial or ability modifications but groups may elect to use the modifications for the closest existing skill.
Example replacement skills include:
Shadowing (the ability to follow someone undetected, and without losing them in a crowd)
Forgery (creating false or duplicate documents)
Acrobatics (various acts of tumbling and jumping)
Contortions (squeezing through narrow spots or reach hard to get things)
Disguise (mask your own appearance or copy that of a certain type of person)
Escape artist (escape any sort of bonds or confinement)
Evasion (if not making attacks, may roll to dodge incoming physical attacks)
Mimicry (may copy and imitate specific voices or sounds)
Tracking (may follow tracks and foot prints)
You could add countless more to the list, and there's no inherent harm in letting the player come up with a few on their own either.