Patch 7.1 added a new craftable 30 slot bag for tailors, and getting it requires a visit to the new Karazhan mythic dungeon.
To start the questchain to learn the pattern, go to Leyweaver Tytallo located in The Grand Promenade in Suramar. He will offer the quest "The Big Bag Theory" if you have completed the entire Imbued Silkweave quest chain, and to complete this you need to head to Karazhan. You need two items, and they each have a 100 % drop chance from Moroes and Viz'aduum the Watcher.
Crafting the Imbued Silkweave Bag requires the following:
- 100 Imbued Silkweave.
- 20 Runic Catgut.
- 10 Blood of Sargeras.