Thieves guilds make more sense if you treat them more like the mafia.With fantasy, I start by never using D&D. I stick the with the feudal system, which requires no explanations. Since I use low-power systems where magic is rare and complex, about the only difference is that large outbreaks of disease can be contained. If Dwarves exist, the wealthy will have access to Roman-era plumbing.
I never use thieves guilds; while it was entertaining for F & Mouser, its a stupid concept.
Given the effect of the arcane and various species, I have included the term 'bravos' which represents individuals who have, or at least claim, skills for the resolution of unusual problems; essentially a specialized mercenary band. There are no guilds or higher organizations of such, just a social niche, much like thief-takers or sell-swords.
I was looking through GURPS' Dungeon Fantasy (which is basically 'play D&D with GURPS rules'), and this bit struck me:
"Consequently, it has some unusual conventions, including a quasi-medieval setting full of modern social developments such as sexual equality, sprawling metropolises, and a cash economy; a technological mix whose only rules seem to be “preindustrial” and “no gunpowder”; casual acceptance of magic and holy miracles; trade guilds for thieves, wizards, and even assassins; and a culture that recognizes “adventurer” as a career choice.";