Much like my work integrating alignment into the game play mechanics, I want to make these adventurer skills part of the overall game for every class.
The traditional thief skills (plus some additional ones) are useable by any class will be unified as adventurer skills (pick pocket, hide in shadows, spot hidden, hear noise, climb etc.). Certain abilities a character chooses when advancing can give them a bonus to test of specific skills.
Philosophically, the Wizard and Fighter classes are specialists in their fields, spending time focusing on learning magic or training for combat respectively. The Expert would have more knowledge of the adventuring skills because it is assumed he used them more to survive over spell or sword.
The difficulty was insuring the Expert did have an advantage over the other two classes in doing these tasks while allowing everyone to do them. I played with bonuses, percentages and other ideas, but they fell flat for me. So using the mechanics from alignment solved the issues and insured the advantage will be given to the expert when using these skills:
The Expert class rolls 2d20 and uses the highest die rolled.
The Fighter class rolls 1d20 and uses whatever is rolled.
The Wizard class rolls 2d20 and uses the lowest die rolled.
I think this keeps the system lean and mean while insuring there is a class advantage for the Expert.
More to come...
(FYI the image is a sketch drawing by Warren Spector, submitted as a possible cover image for Ultima Underworld.)