For my mod (and character) I'm alot busy with the offcial lore of The thieves and the it's leaders so I can expand the Lore myself..

I've some pretty good Idea's concering the origin of the guild wich is quite heavy made up.. but in my opinion still possible... I will only add this lore if there isn't any other (possible) origin....

well there was more then one theives guild actually morrowinds theives guild is not the same one as the one in cyridill

Oddly enough, we have more information on the origin of the Dark Brotherhood than with the Thieves Guild. There is no plot arc in Daggerfall, but I'd try to get my hands on the Morrowind Thieves Guild Arc (and relatedly, the Fighter's Guild because that is the 'official' story) before going ahead, and all of the Morrowind and Oblivion books on the thieves guild and/or the thiefy skill books. You've probably already read most of this, but I'm being thorough for the sake of thoroughness. TIL marks what books raise what.

They're are the same guilds only it has other members and leaders.... just like the fighters and mages guild

No, they are not. Gentleman Jim Stacey's organization was a wholly separate entity from the Gray Fox's organization, complete with different structure and means to reach the top. And http://www.imperial-library.info/obbooks/myth_menace.shtml cites them as separate organizations.

Whereas the Fighters and Mages Guilds in the different provinces are bound to the same codes and rules; they all answer to the same higher authority; The Arcane University for the MG, and the Elder Council(?) w/ the Guilds Act for the Fighters Guild.

Aa.... but that changes some things... I'll be going to rethink some sketches then

I haven't studied my TES lore in a while, but wasn't there a conflict with the two guilds from Morrowind and Cyrodiil?

I remember in Morrowind the guild was known about by most people, sort of public, and there was a lot of talk about how the guild was "big in other provinces" or something, but having a hard time in Morrowind because of the Tongs. Obviously the exact words escape me, but it sounded as if the Thieve's Guild was big in Cyrodiil and was the same thing...

Then in Oblivion people act as if there is no Thieve's Guild, that it is just a myth. I thought that was a much talked about contradiction?

i think youre reffering to the Morag Tong and the Dark Brotherhood. within morrowind itself there is conflict between the thieves guild and the Ccammona Tong.

as for the thieves guild, i dont remember any info on its founding. TIL'd guild page says that its been lead by the gray fox since about 3E100, though i would expect that there have been other guilds of thieves prior to his coming. i would also assume that each region has its own guild, which is more like a confederation of thieves and their fences. thehttp://www.imperial-library.info/zeph/tes_tre_3/t.shtml has some info on this, but it doesnt really go into the foundations of the guilds.

They're likely involved with more than just stealing, think of them as a mafia but w/o whacking. Instead, the rats are framed. The wealthier members are probably decent spellcasters to boot.

That depends on the legality of the activities normally managed by criminal syndicates in the real world, specifically gambling and prostitution. If they're illegal, then some kind of criminal syndicate likely charges them for the right to operate. However, the makeup of temples in Daggerfall strongly suggests that prostitution is, at the very least, sanctioned as a religious practice (even here on earth, prior to the triumph of Christianity, using prosttutes to get funds for the temple was not unheard of). Arena may have gone even further, though I don't know about it. It could be that, in addition to the temple practices, prosttutes also have an official Imperial guild... meaning that instead of the "Mafia" controlling and profiting from the practice, it's the Empire.

Gambling would be the same way.

Chances are, the Thieves Guild has no real origin outside of which Thieves Guild was the first Thieves Guild, thus giving way to future Thieves Guilds, but slightly different from one another in that one may be a secret organization akin to the Chicago Mob, or something more in line TES guild tradition, a loose organization of thieves who function as that "necessary evil" that many nobles tend to rely on.

It should be noted that the Thieves Guild, as a whole, has never really been about being a filthy thief (See: The abolition of murder by the Cyrodiilic Thieves Guild and the Bal Molagmer (sp?) of the Morrowind Thieves Guild) , so it wouldn't be out of the question, I would think, to say that perhaps the first Thieves Guild (which you'll undoubtedly have to make up for yourself) may have set a theoretical honor code for thieves, or may be the first Thieves Guild was a mix of all the different sorts of TG's that ever manifested themselves that split for whatever odd reason.

there was a rumor that the founders of the thieves guild are clones from the skull of corruption. of course that could've just been a story.

The premise of a guild is control, that is its design. While guildsmen are honorable to one another, they are fierce competetors with non guildsmen. Strongarming, blackmail, framing even arranging an accident or two aren't out of the question, but outright murder is. There are the standards the thieves guild professes, and the standards they survive by, I'm sure. There's also no telling how much influence the highest government of a provence has over its thieves guild, it would make for an apt crime deterent and profitable enterprise...

The MG made sure of that in MW about competition. Likewise, the DB actively hunts down free lance and rival assassin guilds. Sometimes they do issue an ultimatum to the assassin they are hunting, to force him in the guild or gamble his life away.

From what I understand, the Thieves Guild exists so that there are some regulation to thieves, like not murdering people in their homes you are stealing from and stealing from each other. Freelancers are looked heavily down upon, if not targeted for "mistakes".

EDIT: Okay, read a good book in DF that talked about the thieves guild. And it basically does regulate the rate of crime. As the author put it, a moderate amount of crime is good, and even lords and nobles approve of their operation. However, when a thief become greedy or stupid, the hammer does come down. By regulating the amount of crime, they keep out unsavory and amateurs out of the way, so they don't disrupt things or make a complete fool out of themselves. Those who accept the invitation to the guild is trained and kept from starving. Not to mention members can be sprung out of jail should things go sour. Now, if someone does decline the invitation to the TG, the freelancer's arms are probably going to be broken horrifically, have a bunch of "accidents" happen to the person, or "an anonymous tip" may come up whenever the person is likely to steal or is on the run.