In case you have missed it, in the BlizzCon 2017 World of Warcraft Q&A panel transcript, you may read that the Backpack will finally get additional slots.

However, there is a catch. You will need to attach a Authenticator to secure your account. You can get one of those from the Blizzard Gear Store (USA/EU or CA,ANZ, LATAM) right away so that once the Backpack size increase goes live, you are ready.

The Q&A clearly states this feature will be available soon, rather than after the Battle for Azeroth launch — which means probably in the first half of 2018. No ETA was provided.

Participant 26: My question is about the default backpack. When will hoarders have more space?

Allen: Members of the Alliance and the Horde: we hear you. We are very close to allowing you to slightly increase your backpack size. One of the things that we want to do is we want to incentivize you to use the authenticator, and so at some future patch, if you have an authenticator attached to your account, your backpack size will increase.