It's now official, Patch 7.3, The Shadows of Argus, will be released August 29, 2017. Here's some information and a few things you can do to prepare for the patch release.

• Class Hall
  Your class hall will still be relevant in 7.3. There will be class hall missions specific to Argus. Some missions will award Primal Sargerite which is the new Blood of Sargeras. This is important for crafting the new gear introduced in 7.3. You will also see gear tokens to upgrade a champion's gear to 925 and new champion equipment.
  
  Artifact Research will continue from 41 to 55 but you no longer have to submit research orders; it will automatically advance by one level each week whether you have logged on or not.


• There will be three new Emissary Quests - Armies of Legionfall (Broken Shore), Army of the Light (Argus), and Argussian Reach (Argus). Some of the world quests have been upgraded to award 875+ gear rather than the current 855+ or 860+, likely a catch-up mechanism.


• Venturing into Argus
  You will be given a quest when you first log in to patch 7.3 that will start you on your adventure into Argus, home of the Legion. There are three regions in Argus: Krokunn, Mac'Aree, and Antoran Wastes. You will want to complete the quest chains in the first two zones in order to unlock the Netherlight Crucible which is the latest form of artifact weapon progression introduced in Patch 7.3. Based on the amount of weapon traits you have unlocked, you can acquire additional abilities and passives from each relic.


• There are two new factions in Argus - Army of the Light and Argussian Reach. As you complete world quests in Argus and some class hall missions, you will gain reputation with these factions. The world quests will also award Veiled Argunite which is equivalent to Nethershards. With the Veiled Argunite you will be able to purchase Unsullied Gear and Confiscated Gear. The Unsullied Gear is similar to Dauntless Gear which is level 880 but it's now BoA which will help you gear up your alts. The Confiscated Gear is level 910 and it includes specific relic types.
  
  Note that you will no longer be able to purchase relinquished gear with Nethershards once 7.3 goes live, so you might want to spend them now.


• Argus will have it's own set of World Bosses. These will start to show up in week two when Invasion Points open. The world bosses have a chance to drop 930 gear.



• What you can do to prepare:
  
  Save up your Order Hall Resources - you will need a fair amount for the new order hall missions and to recruit new, more powerful followers.
  
  Acquire Obliterum. There are currently 9 new pieces of crafted gear. These start at 885 but can be upgraded to 935 using Primal Obliterum and Primal Sargerite at 5 levels at a time. The Primal Obliterum requires one Obliterum and one Primal Sargerite to make. It will also be tradeable on the AH but the Primal Sargerite is BoP. Currently each character will only be able to equip two pieces of the new gear (except for Alchemists, see below) - a head piece made by Jewelcrafters and either a plate chest, leather or mail shoulders or cloth legs. So you will need something like 20 Primal Obliterum per character to upgrade both pieces to 935.
  
  Alchemists should save up their Blood of Sargeras because they will be able to transmute 25 bloods into one Primal Sargerite per day. Alchemists are also able to craft a replacement for their Infernal Alchemist Stone called the Astral Alchemist Stone so they should be able to equip 3 pieces of 935 gear.


• A new raid with 11 new bosses, Antorus, the Burning Throne, will open some weeks(/months?) later with the arrival of patch 7.3.5. Currently on the PTR, the loot from the Normal raid is shown as 930+ to 940+. So, at about the time that the raid is opened, I would expect that we will have a means of upgrading our Legendaries to 1000 and our current, crafted gear to something like 930. This will likely require basic Obliterum rather than Primal Obliterum so, at that time, you will have another reason to stock up on Obliterum.


• Argus is a pretty unfriendly place with mobs almost everywhere, so, if you are somewhat undergeared, I would stock up on healing potions, food and/or bandages.