There are certain quests in the game that gave unique recipes / patterns. Requiring you to do certain quest chains 2-20 quests long to attain the item on the right character, with the right professions.
Legion has a small chance for the Codex rank 3 from bosses and caches from reputations. I think profession specific.
Most things are now do it once and you are done.
If you are a completionist you will still need to do class specific quest chains.
Faction/ Zones questlines.
As certain zones have their own lore for both factions.
I think there are a few more examples (overall) that I cant recall at the moment.
Utilizing the website. Dataforazeroth.com
Will satisfy all your completion needs.
You can add your entire account through a queue system and then see exactly what you havent done on your account.