Here are the tweaks to the Expert NPC class found in the DMG:
- The expert must first pick a sphere of expertise. This needs to be defined narrowly enough to be meaningful; thus one cannot be an expert in "tricks" in general, but one can be an expert in "stage magic" or "animal tricks" or whatever.
- The expert then designates 10 skills to be class skills. Of these 10 skills, 3 are specially designated "core class skills". These 3 skills must directly relate to the expert's sphere of expertise. A "stage magic" expert, for example, might take Sleight of Hand, Bluff and Spot to be her core class skills.
- The expert adds her class level to any skill check involving the 3 core skills.
- At 1st level, and every 4 levels thereafter, the expert gets a bonus feat that directly pertains to her sphere of expertise. Typical examples include the obvious "Skill Focus" feats; our "stage magic" expert could also take "Persuasive" or something similar. Feel free, at DM's discretion, to take weakened versions of class feats if they fall within the sphere of expertise; an expert in "animal tricks" might have an animal familiar as a druid of half level, say, or an expert in "history" might get the equivalent of bardic knowledge on any check involving the area of history in which they are an expert.
- A character may, upon levelling, choose to gain another sphere of expertise. Treat this as a separate class for all effects, including the core skill check bonus which does not stack. Thus a Expert 3 (stage magic)/Expert 2 (salesman) would be a level 5 character with all that that entailed; but if he had taken Bluff as a core skill in both Experts, his class bonus would be +3, not +5.
- Other than that, the Expert class is as written in the DMG.
Why: As written, the Expert class doesn't produce genuine experts in the field. An Expert NPC will, in fact, often be comparable to an ordinary (i.e. non-expert) PC of equivalent level, thus rendering the whole concept of "expertise" somewhat vacuous. With this, Experts will truly be expert, with a mastery that no other class can possibly match -- just as it should be.
Pro: Obvious, really. One additional aim is to make a couple of levels of Expert an enticing option for a PC as well. To do that, the sacrifice of a few levels ought to come with some kind of reward, which I'd like to think this Expert class does quite well.
Con: The one thing I haven't playtested is the addition of the bonus feats. If that makes Experts too powerful, I'd recommend axing the bonus feats but retaining the ability to pillage from class feats (or just be flexible in making stuff up). Personally, I like this the way it is, but YMMV.
My main balance concern isn't the bonus feats, but the expert's ability to add her full class level to any skill check involving core skills. This isn't a problem for a low-level expert, or a PC who dips two or three expert levels, but beyond about Level 5 this starts to provide absurd bonuses - an Exp12 stage magician who's got a Bluff skill check of (+15 ranks +4 Cha +5 feats +_12_ class level) = +36, frex.
I'd instead recommend limiting this bonus to the lower of class level and key attribute, similar to how the Duelist PrC gets a bonus to AC that's the lower of class level and Int bonus. Other than that, this is probably a decent enough power boost for an Expert, assuming you think one is needed.