(Pulling this back down off the asset browser, for sake of consistency. Also fixed formatting.)
 

Thieves Guild didn’t get much at all planned out, so this is easy. I’m just going to go and copy-paste Parker’s notes on it. Link to original topic listed at bottom.

Personality of the Thieves Guild
Real world inspirations for the Thieves Guild: Robin Hood, Hollywood pirates, Prohibition bootleggers, modern street gangs, labor unions.
“Though we cannot condone the actions of the Thieves Guild, we praise their faithful dedication to the Emperor and to Imperial culture.” ¹
The Thieves guild is an interprovincial criminal organization grounded on a fraternal value system. Honor among thieves is the name of the game. Unlike the other Imperial guilds, the Thieves Guild does not have public guildhalls, though they do typically gather in specific buildings in large towns, such as seedy taverns and inns. Disregarding Oblivion’s silly retcon, the public is largely aware of the Thieves Guild’s existence, though perhaps not the extent that they are active in a particular community. Despite its illegality, the Thieves Guild is widely recognized as a distinctly Imperial organization.

The Thieves Guild operates on the premise that crime in moderation is good for the economy. Local authorities across Tamriel tolerate the guild’s presence as a crime regulator in the areas it operates. If you steal on the Thieves Guild’s turf, you either join the guild or get your arms broken² Members can steal at their own discretion as long as they follow three basic tenets.

1. Never steal from fellow guild members.
2. Never kill anyone on the job.³
3. Don’t steal from the poor.

Typical Thieves Guild activities include pick-pocketing, burglary, smuggling, setting up markets for stolen goods, creating informant networks and bribing public officials.

¹ For My God and Emperor – The Imperial Cult Faction Book.
² Confessions of a Thief – Daggerfall-era book about the Thieves Guild.
³ Of course, exceptions are made under extreme circumstances. See “The Brothers Ieneth” and “Kill Hard-Heart” quests from vanilla.

The Thieves Guild in Morrowind
The Thieves Guild has always been an underdog in Morrowind. It followed the Empire into the province, setting up shop in Imperial colonies before beginning to make inroads into native Dunmer settlements. It immediately met stiff competition from the Cammona Tong, Morrowind’s entrenched native criminal syndicate. An intense gang war between the two groups is currently raging across the province, and most acknowledge that the odds are stacked against the Theives Guild. They’re the relative newcomers after all. The Cammona Tong not only has strong economic and cultural ties to their native homeland, but also powerful political backers in Great House Hlaalu. They are also adept at using ruthless tactics that the Thieves Guild tends to shy away from.

“The Camonna Tong don’t mind killing people. Heck, they LIKE killing people. The Thieves Guild, on the other hand, thinks killing is bad for business.”⁴

Faced with a resourceful enemy, Morrowind’s Thieves Guild might have to sacrifice its nonviolent ideals simply to survive. The other Imperial factions are in no place to assist it, and some, such as the Fighters Guild, are outright hostile.⁵ The Dunmer Great Houses and the Temple would likely love to see the outlander thieves all dead. The only groups in the game that have positive disposition/reputation modifiers with the Thieves Guild are the Imperial Cult and the Mages Guild.⁶

A few years ago, the Guild’s previous charismatic leader, Arnie the Scrib, retired from Guild activity and went into hiding as the condition to a fragile truce with the Camonna Tong. This truce, which the Camonna Tong never intended on keeping, has since fallen through, and the gang war has erupted once more. It will be up to the player to rise to the Guild’s top leadership positions, combat the Commona Tong, and root out internal threats, which may or may not be related to Vaermina’s Skull of Corruption falling into the hands of the Guild’s current leader, who is steering the Thieves Guild towards disaster.

⁴ Honor Among Thieves – Thieves Guild Faction Book.
⁵ In the Fighters Guild’s case, this is due to Camonna Tong influence.
⁶ Naturally, the Thieves Guild is still willing to target the Mages Guild, as seen in the “Loot the Mages Guild” quest given in Ald-ruhn.

Hierarchy of the Thieves Guild
I. Master Thief
Leader of the Thieves Guild. The most skilled Thief in the province
Endoroth (current) Narsis
Arnie the Scrib (former) Teyn

II. Mastermind
Regional Thieves Guild leaders located in major cities. Take orders from the Master Thief, but generally act independently.
Almalexia [a yet-to-be-named theater]
Gentleman Jim Stacey Vivec Simine Fralinie: Bookseller
Old Ebonheart
Baan Malur

III. Ringleader
Local Thieves Guild bosses located in large towns. Officers in major cities above.
Silniel the Maven Firewatch Howling Noose
Greedy Hofmund Helnim Falkoth’s Clothier
Tear
Andothren
Aengoth the Jeweler Ald-ruhn The Rat in the Pot
Sugar-Lips Habasi Balmora South Wall Cornerclub
Big Helende Sadrith Mora Dirty Muriel’s Cornerclub
Kartur
Kragenmaar

IV. Captain
Work under Masterminds and Ringleaders as lieutenants.

V. Bandit
High rank Guild enforcers

VI. Operative
VII. Blackcap
VIII. Footpad
IX. Wet Ear
X. Toad

Notable Guild Leaders
Endoroth (placeholder name)
Gentleman Jim Stacey
Arnie the Scrib

My idea is to make the Thieves Guild’s current leader a wily Bosmer, if only so we can use Ashtaar’s awesome Bosmer head (which Haplo ha said should be reserved for a unique NPC) on someone notable. He’s a cunning, moderately amoral con-artist who is willing to use ruthless tactics to further the Guild’s goals and combat the Camonna Tong. He’s also allowed more corruption under his watch (as much as a criminal organization can be corrupt). As mentioned above, he might be under the influence of a pilfered Daedric artifact.

Gentleman Jim Stacey is the Master Thief of the Thieves Guild in vanilla. In this plan, he is still the ranking member on Vvardenfell. Jim is more in line with the Thieves Guild’s idealistic side, as seen in his Bal Molagmer quest line. He’ll probably be opposed to the current Master Thief’s actions, but will be unable to do anything about him without the player’s help.

Arnie the Scrib is the former leader of the Thieves Guild in Morrowind. He is a gentleman thief, skilled counterfeiter and the legendary author of the in-game book, “Honor Among Thieves.” He currently resides incognito in the Imperial village of Teyn, under a false name. At some point during the quest line, the player will find and convince Arnie to return from retirement, and help expose the current Master Thief’s wrongdoing. Afterwards, he will declare the player the new Master Thief.

Gameplay Notes
The Thieves Guild is one of our mod’s three core factions:
“These core factions' quest-lines do not at any point conflict with any other faction's quest-line, including each other. They contain a central narrative, but also many superfluous quests. The function of these factions is to provide players with appropriate skills easy access to structured quests in all parts of the game world. Participating in them should not very strongly define the player's character, politically, socially, or ideologically”⁷

Besides its role as a source of quests, the Thieves Guild provides the useful service of removing bounties from player’s heads for a price. This makes it an especially attractive faction to join.

⁷ Sload in his “Ranking the Factions (and their conflicts)” thread. I do not know what his comment on factional conflicts will mean in regards to the current faction conflict between the Fighters Guild and the Thieves Guild on Vvardenfell

Finally, a small quote out of the old Master Plan:
-crime: ownership assignment and prison distribution are the main things to consider for level design, and the former is pretty straightforward. Providing the player with incentives to commit crimes, such as looting vaults in which case the incentive is generally, of course, the loot, is worth encouraging. The Thieves Guild pretty much exists for that purpose, but other factions and miscellaneous quests can also require the player to commit a crime, though often, even in the Thieves Guild, there will be alternate ways of completing quests which do not require the player to commit a crime.

REFERENCES BELOW THIS LINE……
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Notes on Thieves Guild by Parker http://tamriel-rebuilt.org/old_forum/viewtopic.php?t=24363
Parker’s Notes in PDF form: http://tamriel-rebuilt.org/sites/default/files/assets/Notes%20for%20the%...
Skull of Corruption http://tamriel-rebuilt.org/old_forum/download.php?id=21833
Old Master Plan https://docs.google.com/document/d/1j8hxOgjA4_A__J2HsV6ANqgTvZIOPQhbtBCukFDNJS4/edit (link is external)
Old plans on the Empire faction. Unfortunately, no mention of Thieves Guild. http://tamriel-rebuilt.org/old_forum/viewtopic.php?t=24271