One houserule I've been working on is advancement by Thieves' Guild. Rather than tie the skill advancement to alignment, it would be based on the flavor of the guild. Want a guild that is notorious for breaching the highest security areas in your world? Done. All the GM has to do is choose:
- 4 Skills to receive the highest bonus
- 4 Skills to receive the second highest bonus
- 2 Skills to receive the second lowest bonus
- 2 Skills to receive the lowest bonus
- One of the two advancement tracks for Casting Spells from Scrolls
Of course, basing advancement on guild flavor opens up a world of possibilities. For one, having a guild with such influence on a character means a direct source for adventure. It also provides opportunity to completely customize the Thief class. In exchange for a smaller selection of skills, perhaps the player advances faster or higher in their areas of expertise. Maybe each guild has a wholly custom list of skills and abilities or favored weapons. With all of these options, I find it best to start with the the fiction and worry about how to make it work within the rules after the fact.