Thieves guild are the pre-Mafia organized crime organizations. They both organize and control the activity of thieves (through intimidation, control of fences, and violence) while using their position of power to their advantage in dealing with legitimate society (deals with guards, bribery of officials, protection rackets, etc.) They undoubtedly do exist, especially in Ul'dah, and could have been interestingly tapped into for game purposes. I only question whether or not one may exist in Limsa, because in Limsa the government itself is essentially an expanded organized crime enterprise and may have muscled out their competition - this may also play into the storyline, like pirates-gone-good, so have the thieves. But... they don't really seem to want go anywhere near villainous PCs anyway...
Finally a second class I may want to level!
Finally a second class I may want to level!