The Broken Isles are unique among WoW continents in that you can progress through zones in any order you want. Once you have made it through the initial quests in Legion, you are free to move about four of the new zones — Val’sharah, Azsuna, Stormheim, and Highmountain — as you wish, with enemies, quests, and rewards scaling to your level. The fifth zone, Suramar, is mostly locked away until you’ve reached a certain point in the story.
The most recent run I did on my hunter, I never set foot in Ghostlands, nor did I need to fully complete Dustwallow Marsh. There are a LOT of quest lines in each of these zones that might not be available unless you progress and complete story-line quests in order. Northern Barrens and Andorhal are especially bad in this regard, but many zones you can easily miss out on a lot of quests if you don't do a vital quest. So make sure you keep an eye on the minimap for quest ! indicators.
Edit 20 It’s been a week since this post and a lot of people are still reaching out to me with questions which is fine. I have had a few different people reach out to me to help make YouTube videos based off this guide and more info in BFA which I’m more than willing to do. Whoever gets the videos up first I will link them here for you guys to enjoy.