The guild design system gives a wealth of detail on the traditional (in the AD&D® game) town or city thieves' guild. However, there are other possible guild structures which can be used by the DM in the campaign world. These are unlikely to be suitable for PC thieves (in most cases), but they add diversity and spice to any campaign.
The Traveling Guild[]
A group of traveling thieves, who work as a body and are effectively a guild on the road, makes an interesting encounter. Three variants on this theme include:
Gypsy Folk: This group travels in horse-drawn wagons. The community is just that—complete with many sniffling children, goats, snarling dogs, a few chickens kept for eggs, and lots else. In towns, the gypsies sell dried herbs and pressed flowers, and the wise woman of the group may read fortunes (using the Fortune Telling/Astrology proficiencies if appropriate). The gypsies will be extremely loyal to each other, and will usually be of Neutral (or chaotic neutral) alignment. If such folk are a common sight on the roads of the land, then the gypsy people may have extended clans related by intermarriage, so that if any violence is done to one of them an entire horde can be assembled to track down and punish the offender.
River Folk: A variant on the gypsy theme is to have a group (or groups) of traveling folk who work the waterways of the country. These people will almost certainly have merchant interests, or convey cargoes for others; thieving would not be an adequate income for them. They will supplement their income with thievery in and around the rougher ports, usually inland or estuarine. They are significantly more likely to be of evil alignments than are road-travelers. In the campaign world of Oerth (Greyhawk), the Rhennee folk are a good example of such river folk.
The Circus: This is a somewhat hackneyed theme, but should always be used in a campaign at some time. Thieves can make up the bulk of a traveling circus, which comes to towns and cities and cheerfully robs them. If the circus has monsters on show, then there is a rationale for having other character classes along for the ride who help the thieves and give the NPC band some diversity. Thus, clerics can be on hand to snake charm and speak with animals while mages could charm monsters or speak with monsters (if of high enough level; arguably a 12th-level mage has lots better to do than traipse around with a circus). Thieves with specialist skills such as acrobats, cat burglars, etc., will fit well into such a group.
Players with any degree of gaming experience will have learned to keep well away from circuses. Old tricks such as having evil, high-level NPCs polymorphed into monsters or animals are well-known to such devotees of the game. So, it may be more enterprising to use some other group of entertainers, such as a traveling troupe of actors and musicians. They can perform the equivalent of passion plays, act and sing charming ethnic curiosities (especially elves), sing madrigals, and the like. A group of light-fingered expert thief choir elves could make for a lot of fun (they look so sweet and innocent, how could anyone suspect them?).
Piratical Guilds[]
Also travelers in some fashion, a guild of thieves who spend most of their time engaged in piracy is a complex and shifting structure requiring careful thought by the DM. There will obviously not be any form of guildhouse for such thieves, although a small number of secret guild hidey-holes (caves in the cliffs, deserted coves, desolate gull-haunted islands) could exist where spare equipment and vital emergency supplies might be placed at strategic locations. guildmasters could exist, but much more likely is a loosely-affiliated structure where several senior thieves, maintaining their positions by force in most cases, are equipotent. They might well all call themselves guildmaster! Such a guild would have a very strong tendency towards chaotic alignments, and a nearly-equal tendency towards evil.
Some type of organization and regularity would have to be imposed to make this a guild at all. Meetings half-yearly, with a quorum for votes and decisions to have any binding quality at all, would be a possibility. These could take place in the major piratical port, perhaps a town or city of unparalleled iniquity (what a place to send some PCs to track down the wicked pirates and stop their evil trade in pressganging/slavery/drug smuggling, etc.). Certainly, some mechanism for obtaining equipment and training (at the least) must exist.
For a notably more structured and stable rulership of a piratical-type operation (slaving), see the adventure module A1-4, Scourge of the Slavelords.
The Guild of Honorable Gentlemen[]
As a really unusual guild, which could exist in parallel with the established thieves guild in a large city, this is a splendid option.
The members are aristocrats, men of learning, education, and exalted social position (preferably by birth). They are refined, dignified gentlemen of impeccable manners and superior Charisma. They are a small group, and they thieve for the excitement of it all. They are bored with their life of sybaritic self-indulgent possibilities, easy comforts, and gold-digging members of the opposite sex. They steal for the thrill of it.
What motivates these people is a challenge to their skill. They steal things not because they are valuable or important, but because they are there to be stolen (which doesn't mean that they are valueless or banal—they usually aren't). The Emerald of Kummkqvaat will be stolen by such a thief not because it's worth a fortune, but because it is believed to be impossible to steal it.
Such thieves will usually be of notably high level, and they will tend to have exceptional Intelligence and Charisma scores. They will very rarely be Evil, and will also tend away from Chaotic alignment. For these reasons, they can become excellent mentors for a PC thief if this is appropriate to the campaign—for example, a PC thief can come across one of them at work, or find evidence of the person's true identity as a renowned master thief.
The Good-Aligned Guild[]
This is a rarity. It has to be. Remember the Player's Handbook definition of a rogue; someone who feels "that the world (and everyone in it) owes them a living." Thieves are "the epitome of roguishness." Most thieves want to do as little work as possible and live as well as possible off the efforts of others. This is not exactly a definition of good alignment. The major problem with a good-aligned guild is simple: The large majority of thieves are not of good alignment. If a good-aligned guild comes into being (e.g., a good-aligned guildmaster comes to power) then many, if not most, thieves will actively seek a non-good (preferably neutral) guild if the guildmaster seeks to impose the values of good on them.
What the intelligent good-aligned guildmaster will do is not to impose or accentuate the values of good within the guild. Rather, he will quietly squeeze evil thieves out, put them at risk, and try to make sure that evil activities (slavery, etc.) are downgraded or made to fail. Eliminating evil is much smarter than trying to establish good.
However some good-aligned guilds can certainly exist in the campaign world. A classic example is the "freedom fighter" guild, a guild which stresses the values of chaotic good, in a Lawful Evil country or state. Such a guild will attract thieves of CG, NG, and CN alignments, and pure Neutrals will go along as usual. Even chaotic evil thieves might join, hating the repressive lawfulness of the state (especially if the guild rulers have the sense to play up Chaos and freedom in their pitch). Such a guild is one with the classic Robin Hood (robbin' hoods?) spirit. It could exist "underground" in an evil land such as the Scarlet Brotherhood lands (in Oerth) or Thay in the Forgotten Realms (FR6, Dreams of the Red Wizards), a secret urban organization with underground, hidden meeting-places and fearful helpers in high places—a superb setting for intrigue. A rural equivalent, with scout-type thieves allied with tough rangers and others combining their skills to chip at the edges of a strong evil state, is another example, more suited to players who like lots of combat and tactical skirmish gaming than political intrigues and tense urban chases and the like.
Other good-aligned guild possibilities exist, certainly, but require more careful thought by the DM. They are possible in a fractionated or oppositional guild structure, in a country where the good/evil division mirrors or parallels some other (e.g., good-aligned elves and predominantly evil-aligned humans), and possibly in super-goody-goody nations where they exist as security consultants and the like (but how do they accumulate their experience points for practicing their skills in earnest?).