In this thread, I will preview a few parts of this update and you'll be able to discuss it with us! More screenshots might be added.
Player mall:
The number of NPCs has been highly reduced thanks to the multivendor system, we now have a single vendor for Weapons, PvP/PvE gear, etc.
But more than reducing the number of vendors, this has allowed us to properly sort NPCs such as the Gem Vendor or the Demon Trainer based on gem color / pet type. We also removed many items that were undoubtedly useless on vendors and kept the useful/situational ones.
All the Class Trainers have been replaced by a Universal Class Trainer. You no longer need to find which one of these trainers is yours, there is just one. The Dual Spec feature will be on this NPC as well.
Small detail, this NPC changes its morph based on the class of the last player who talked to him.
Here's a little after/before album for two new NPCs on the mall: https://imgur.com/a/mZchx
An NPC to trade currencies is finally part of the mall (Badges of Justice or Arena Points into Honor Points, and BG Marks into AV ones).
Many other NPCs have been removed/replaced. It freed so much space on the mall that we decided to add a small "Appearance Area" which contains the transmogrification NPC, the Special Transmog Vendor, the Race Change NPC and the twins that sell the Legendary Models (renamed from "Legendary Transmog").
Aerial preview of the new mall. (It still needs finishing touches):

Profession area:
We decided to remove the reputation barrier for professions, and thus Quel'danas and Shattrath areas will be phased out.
Many suggested to migrate the trainers to the middle section in Tanaris (between the Player Mall and the Battlemasters Area), and this is where the new profession area will be!
If we randomly target players around Tanaris, it is rare to see someone without a flask or any other consumable. There is definitely no reason to force players to do something that is completely at the opposite of what SF players should be focused on (PvE or PvP). I'm not saying players shouldn't devote time to grind reputations if they want to, but we shouldn't force them to do that for an aspect of SF that doesn't involve PvP/PvE and even ends up afk-crafting. On this point few suggestions have been made, even a very serious one from Bucovsky (world PvP to gather materials) but the pros didn't outweight the cons.
We may add a NPC that directly sells most of the consumables for honor, for those who do not want to waste time.
The custom Nat Pagle is on the oil rig as well, it makes sense considering that its daily quests are fishing/cooking related.
Screenshot of how it should look like:

Season 4 and Arena:
Even though the population isn't as high as it used to be, BGs are still running. However this loss of population over the last year is affecting the arena scene. We would like to see more enthusiasm from the casual players in playing arenas. It's a sure thing many would like to try and play few games, but they either do not find a mate or get smashed by a team against which they do not stand a chance.
We need to give a reason for casual players to try arenas and improve their gameplay, reducing the cost of many S4 items may be one. Note that every in-game currency follows a pattern which we can't change. So we are setting the costs from the available patterns and it isn't possible to set a custom price. These might be the new requirements to get S4:
*Relics/Wands/Throwns: No rating & 800 Arena Points
*Rings/Necklaces: No rating & 900 Arena Points
*Bracers/Belt/Boots: No rating & 1000 Arena Points
*Gloves: 1550 rating & 1875 Arena Points
Chest: 1600 rating & 1875 Arena Points
Legs: 1650 rating & 1875 Arena Points
*Head: 1700 rating & 1875 Arena Points
One-Handed Weapons/Shields/Off-Hands: 1950 rating & 1304 Arena Points
Two-Handed/Ranged Weapons: 2000 rating & 1500 Arena Points
*Shoulders: 2050 rating & 2625 Arena Points
Items preceded by a * are those whose price has been changed.