Several new additions were added to Brawler’s Guild including twelve new boss fights. You’ll need to loot a specific item before you can purchase the challenge cards for each optional boss.

Item Boss Requirement
inv misc flute 01 Brawler's Item Guide Knockoff Grumplefloot Rank 4
Grandpa Grumplefloot		 Dance with Grandpa Grumplefloot in The Grummle Bazaar, Kun-Lai Summit.
Reward: Floot-Tooter’s Tunic
inv misc noose 01 Brawler's Item Guide Hozen-Fur Fuse Rank 4
Master Boom Boom		 Complete the The Burlap Grind daily in Kota Basecamp, Kun-Lai Summit and start killing Hozen until the item appears in your bag.
Reward: The Boomshirt
inv weapon hand 30 Brawler's Item Guide Raptorhide Boxing Gloves Rank 4
Ty’thar		 Purchase some Raptorhide Boxing Gloves from the Darkspear Rebellion Quartermaster in the Barrens for the cost of one Radical Mojo.
Reward: Undisputed Champion’s Shirt.
inv misc bone 08 Brawler's Item Guide Impeccably Sharp Tooth Rank 5
Razorgrin		 Fish up an Impeccably Sharp Tooth from any pool in Pandaria.
Reward: Sharkskin Tunic
trade alchemy potionb5 Brawler's Item Guide Vial of Reddish Ooze Rank 5
Splat		 May be obtained from your Greater Cache of Treasures (random scenario reward) or Surplus Supplies (Barrens caravan escort).
Reward: Ooze-Soaked Shirt
inv egg 04 Brawler's Item Guide Frost-Tipped Eggshell Rank 5
Dippy and Doopy		 Battle Doopy (25 elite penguin) in the northern tip of Icecrown (47,14) to receive a Frost-Tipped Eggshell.
Reward: Tuxedo-Like Shirt.
inv gizmo 03 Brawler's Item Guide Modified Chomping Apparatus Rank 6
Mecha-Bruce		 Purchase the Challenge Card for Bruce (after you’ve defeated him in Rank 1).
Reward: Brucehide Jersey
inv gizmo khoriumpowercore Brawler's Item Guide Dusty Old Robot Rank 6
Blingtron 3000		 There’s a chance to loot a Dusty Old Robot from your daily Blingtron 4000 gift package.
Reward: Last Season’s Shirt
inv misc book 02 Brawler's Item Guide The Bear and the Lady Fair Rank 7
The Bear and the Lady Fair		 May be obtained from your Greater Cache of Treasures (random scenario reward) or Surplus Supplies (Barrens caravan escort).
Reward: Gorgeous Blouse.
inv stone sharpeningstone 04 Brawler's Item Guide Paper-Covered Rock Rank 7
Ro-Shambo		 There’s a chance to receive a Paper-Covered Rock from harvested crops on the farm.
Reward: Paper Shirt.
inv misc shovel 01 Brawler's Item Guide Digmaster’s Earthblade Rank 9
Mingus Diggs		 Complete Tol’vir digs in Uldum to receive Digmaster’s Earthblade. You can also hand in Restored Artifacts and convert Spirit of Harmonies for more Tol’vir boxes.
Reward: Digmaster’s Bodysleeve
inv belt cloth panstart a 01 Brawler's Item Guide Well-Worn Blindfold Rank 9
The Blind Hero		 May be looted from the Darkmoon Game Prize after completing each daily at the Darkmoon Faire. Note: you will only be able to see the item if you are 90. Farming the item on lower level characters may still be done if you send the boxes to your Brawler character to open.
Reward: Sightless Mantle.

Once you have defeated all of the bosses above, you’ll receive the Wraps of the Blood-Soaked Brawler for Deck Your Collection.

I managed to loot the Dusty Old Robot from Blingtron a couple of days after the patch launched so I was eager to see what the quest was about. From there I wanted to work on collecting the others so I did some research and referenced them here. Quite a few were fun to collect as opposed to others which took some time and a lot of luck. The Hozen item took about 30mins while two came from the daily scenario bag as opposed to the caravan escorts. Sadly, the last item I needed was collected on an alt (two days in a row) while harvesting her crops but tonight I managed to collect the item from harvesting a normal crop.

Undisputed Champion shirt

So far I’ve defeated four and tried a couple of others but my question is now, how am I going to hold all these extra shirts?