Professions in Battle For Azeroth will be significantly different than how they worked in Legion. This page provides an overview of the new items and the professions changes in Battle for Azeroth. I will update this page everytime I find something interesting in the new beta builds.
Patch 7.3.5 introduces a new scaling system similar to what we have in the Legion zones, so every mob will scale to your level now in older zones too. Fortunately, the scaling is capped at certain levels for each expansion, you won't be fighting level 110 mobs at Barrens.
So if you farm old materials with a level 110 character, this change shouldn't effect you since you will one shot everything anyway. However, this will make it harder to farm with lower level characters.
Skinning and Tailoring
Based on the tests I did on PTR, there was no change in cloth or leather loot tables. So, even if the mobs scale up to a higher level, they will still drop the same leather and cloth as before. The level 58-60 mobs in Barrens still dropped Light Leather and Linen Cloth.
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:
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
To make it easier to level professions to 800, most recipes will stay orange for 20 more points. This is a huge change! For example, the blacksmithing 780-800 part needed around 120 Demonsteel Bar and 60 Blood of Sargeras at minimum to craft 30 Waistguard. (you probably needed to craft 50 if you were unlucky). But now you will only need 20 Demonsteel Bar and 10 Blood of Sargeras to craft 5 of them.
The rank 2 recipes have the skill ranges that the rank 3 used to have, so if you are really unlucky with the rank 3 Alchemy recipes, now you can level Alchemy to 800 with just the rank 2. Or you can level inscription to 800 without waiting for the Darkmoon Faire.
It seems engineering helms are the only one that didn't change because the helms were already yellow up until 800, so engineering was already easier to level than the other professions. But now with this change you actually don't even have to make helms, because you can make the previous recipe up to 800.