First off this is what you do after the Thieves guild main quest line…
Vex and Delvin give out sidequests (jobs) all over Skyrim. There are Four main capitals you want to worry about, Solitude, Windhelm, Whiterun, and Markarth Goto Vex and Delvin and choose any job you prefer but you only need to complete Five in each of those Four cities.
Each time you have done Five in any one of those particular cities Delvin will spawn a special mission for you in that city to restore the Thieves Guilds honor there. Do that mission and stop accepting mission to that city while concentrating on the remaining cities, til you have completed all Four of the special missions.
You will then be presented a special ceremony in the Cistern declaring you a Master Thief
Congrats you have restored the Thieves Guild to its former glory.
Update: CAN be completed before main story line is finished
Vex and Delvin give out sidequests (jobs) all over Skyrim. There are Four main capitals you want to worry about, Solitude, Windhelm, Whiterun, and Markarth Goto Vex and Delvin and choose any job you prefer but you only need to complete Five in each of those Four cities.
Each time you have done Five in any one of those particular cities Delvin will spawn a special mission for you in that city to restore the Thieves Guilds honor there. Do that mission and stop accepting mission to that city while concentrating on the remaining cities, til you have completed all Four of the special missions.
You will then be presented a special ceremony in the Cistern declaring you a Master Thief
Congrats you have restored the Thieves Guild to its former glory.
Update: CAN be completed before main story line is finished