|Maybe not Chaotic Good...|
Fictional guilds take a leaf from Fritz Leiber's 'Ill-Met in Lankhmar' and 'Thieves' House'. Other trade guilds (locksmiths, masons, tailors) had stringent rules and protectionism - the model for Cervantes' Riconente and Cortadillo - a satire on guild stratification and nobility. Dickens reveals the darker side of a gang in Oliver Twist. When the truth outs about his unwitting involvement with pickpockets, Oliver barely enjoys liberty before being beaten and dragged back to Fagin's kitchen. Ali Baba fears the Forty Thieves who object to his own theft by trying to kill him. None of these behaviours are benign.
Medieval thieves' guilds weren't plentiful in reality. Extended slum gangs like Cour des Miracles (inspiration for Hugo's The Hunchback of Notre Dame and Les Miserables) were unusual with formal cant disseminated by student malcontents. Dekker's compilation of roguish cant skates the latter edge of most fantasy game timeframes. An informal language evolved over time out of necessity. Gangs and syndicates, as Tourq notes, are more interesting than the generic thieves guilds. They provide some inspiration in defending turf (consider most street gangs) and dues paid into a hierarchy (like the 'Ndrangheta) that grants permission to operate. A guild may have it's own character, identifiers and safe houses. Here is an opportunity to add encounters, plot hooks, NPCs. Handwaving to a 10% cut and access to thieves' tools is missing an opportunity.
One particular cheesy trope are the romanticised 'do-good' guilds. Figures like Autolycus or Robin Hood are meant to be at odds with the norm, not forming fantasy Rotarian clubs. It may be a conceit of working where gods have local franchises, paladins detect evil and weak wizards use ESP. Other factors include literary tropes like friendly spies inspired by David Eddings et al. Sadly, a profit-making organisation based on illegal activities and stealing made up of career criminals will not attract nice people. Consider the morals of people like Al Capone or The Krays. Nor do they blithely accept newcomers without some test of worth.
Guilds are organisations worthy of respect, particularly if they're badass specialists like assassins. You might be looking at local groups like the Sons of Anarchy or the Sopranos with ties to other organisations. If there's a big enough membership or influence, the guild may resemble one of the Mafia families. Any fantasy game will need to consider how magic affects their operation, taking a leaf out of Ill-Met in Lankhmar may mean a 'tame' spellcaster. There are limits of course - asking and answering these questions is relatively quick and offers interesting material. There are also plenty of good sourcebooks featuring criminal guilds. Either option beats just having a generic guild!