There's actually more to the Thieves Guild, you just have to unlock it. Here's how.
Vex and Delvin offer special thieving jobs. Vex has four types and Delvin has three types. They are randomized, and usually just involve going to X location and stealing Y object. There are an infinite amount of these jobs. However, you will need to do some of these in order to unlock the final Thieves Guild quests.
Once you've done approximately 5 or so randomly generated Thieves Guild quests in any specific city - Solitude, Markarth, Windhelm, or Whiterun, you may talk to Delvin (this is important, ONLY Delvin offers the special tasks, not Vex) and he may offer you a special task. The special tasks are 4 specific quests to boost the Thieves Guild's reputation in those 4 cities. Note that Vex and Delvin will offer quests for Riften as well - these are useless, other than giving you money, and if you just want to finish the questline you can safely ignore them.
To recap - you want to do enough random jobs in each of the 4 other major cities (Solitude, Markarth, Windhelm and Whiterun) and keep checking in with Delvin to see if he has a special task for you.
Each time you finish a special task, you'll get a new fence in that city, and you'll get a new merchant in the Ragged Flagon. The Guild itself will also start to appear more habitable, with throw rugs, more lights, fancier furniture, food, and the like appearing. There will also be new NPCs and new recruits walking around.
Halfway through the special tasks Tonilia will ask you to do her a favor and deliver some moon sugar to a Khajiit Caravan. Do this, and you can then use any Khajiit Caravan you encounter as a fence.
Once you've done all the special tasks and deliviered the moon sugar, you'll be told to see Brynjolf about becoming Guild Master. Simply talk to Brynjolf and he will make you Guild Master. At this point you will get 4 rewards:
1 - Tribute Chest Key
This opens a chest by the Guild Master's desk, where the guild will periodically place free money and other treasures for you.
2 - One With The Shadows achievement
"Returned the Thieves Guild to its Former Glory"
3 - Amulet of Articulation
This is a necklace that makes persuade checks "almost always succeed" and grants +10% Speechcraft
4 - Guild Master Armor, Gloves, Boots and Hood
Armor increases carrying capacity by +50
Boots increase pickpocket success by +35%
Gloves increase lockpicking by 35%
Hood makes prices 20% better
The Guild Master armor looks like what Mercer Frey was wearing
http://cloud.steampowered.com/ugc/648745786628593733/1F660FB3C68130E4AD88B40F59626552B4487A2A/
Note #1: I do not know if this armor is leveled loot or not, but I was level 40 when I got it. These stats may only accurately reflect the gear at level 40 or might be unique, I don't know.
Note #2: You do not have to finish the 24 Stones of Barenziah quest to fully restore the Guild.

