Patch 7.3 adds new crafting materials for every professions, and if I remember correctly, this is the first time new base materials were added to the game this late and not just at the start of the expansion. You should expect that these will be really expensive at first.
These materials are:
[Astral Glory]
[Empyrium]
[Fiendish Leather]
[Lightweave Cloth]I made farming guides for all of these new materials so check them out if you want to farm them. But keep in mind that most of these farming places will probably be pretty crowded, since everyone want these new materials
General changes
- Crafting professions are receiving several new ilvl 885 crafted items that use new Argus materials.
- You can use
[Obliterum] and 
[Primal Sargerite] to craft 
[Primal Obliterum]. This item is used to upgrade the new crafted armors to 935.
NOTE: You can only upgrade the new items to 935, the old crafted item's ilvl still stays at 900.
- [Primal Sargerite] is the new
[Blood of Sargeras]. You can get it by Alchemy transmutes, Argus World Quests, and gathering professions on Argus. Later on you can also get it with the 
[Boon of the Lightbearer] enchant, but you will need Revered with the Army of the Light faction before you can buy it. This enchants allow you to sometimes loot 
[Light's Fortune] which can contain Primal Sargerite.
Unlocking new Argus recipes
To unlock the new recipes for Blacksmithing and Leatherworking, you have to complete the Argus story quests up to 
The Child of Light and Shadow. You can see the full quest chain at this link, but you will automatically get a quest called The Hand of Fate to start this quest chain as soon as you long in when 7.3 goes live. After you completed the quests, you will get a quest at the ship for your profession, which unlocks the recipes.
For Alchemy, you can get the quests at Krokul Hovel after you follow Velen and meat Chieftain Hatuul.
Jewelcrafting and Tailoring quest will unlock on week 2 (september 5) once you finish the The Path Forward quest at Mac'Aree.
Profession Changes
Alchemy
- New trinket:
[Astral Alchemist Stone] - New potions:
[Lightblood Elixir], 
[Tears of the Naaru], 
[Astral Healing Potion] - New Transmute: [Transmute: Primal Sargerite]
Blacksmithing
- New plate chest:
[Empyrial Breastplate]
Cooking
- New feast:
[Feast of the Fishes].
It's currently not know what buff it will give, but it might be this one:
[Well Fed] - Transform into a fish. When underwater, you can breathe water and swim rapidly. When on land, you can only flop around, and you will gradually lose health.
Enchanting
- Finally the recipe
[Formula: Chaos Shatter] was added to break down 
[Chaos Crystal] into 
[Leylight Shard]. Requires Argussian Reach Friendly reputation.
Engineering
[Gravitational Reduction Slippers]
[Wormhole Generator: Argus]Fishing
[Corbyn's Beacon] - reward for Helpful Friend reputation with Corbyn.
[Trawler Totem] - reward for Good Friend reputation with Akule Riverhorn.- New mount:
[Pond Nettle]. Don't know much about this one yet.
Inscription
[Vantus Rune: Antorus, the Burning Throne]
[Glyph of Dark Absolution] - Drop from Seat of the Triumvirate.- [Glyph of the Voidling] - Drop from Ataxon.
- [Glyph of the Lightspawn] - Army of the Light - Friendly
- Four paladin Glyphs to change the look of
[Divine Steed].
Jewelcrafting
[Masterful Argulite] - Raw gem: 
[Argulite]
[Quick Lightsphene] - Raw gem: 
[Lightsphene]
[Versatile Labradorite] - Raw gem: 
[Labradorite]
[Deadly Deep Chemirine] - Raw gem: 
[Chemirine]- 4 new head armor:
[Empyrial Cosmic Crown], [Empyrial Deep Crown], [Empyrial Elemental Crown], [Empyrial Titan Crown]
Leatherworking
- New mail and leather shoulders:
[Fiendish Spaulders] and 
[Fiendish Shoulderguards]
Tailoring
- New leg armor:
[Lightweave Breeches]