Stormheim is the zone you want to start it. For some reason Legion treasures are not worth any experience like they are in WoD...except in Stormheim. I haven't been able to find out any reason for this, and it's possible it may get fixed/nerfed at a later date. But for the time being you REALLY want to jump from treasure to treasure to augment your quest experience.
Fourth: When the enemy faction controls the Warfront, you get access to 11 quests that allows you to trade materials for AP and honorbound rep. This is huge because you get 750 AP per turn in and 500 rep per turn in which gives you a total of 8250 AP and 5500 rep; that is a massive amount. All the material you got from scrapping all your leveling gear will come in handy here and I imagine that you will be able to trade other players for different turn ins. I highly recommend trying your best to do all 11. Also, this will be a great opportunity to make some serious cash for your profession because people will be buying the same items in bulk.
First up we have patch 7.3.5, which was implemented several months ago as a major shakeup to how leveling was approached. Instead of going from low to high level zones like the way the game had always operated, Blizzard opened up the map with more of a Guild Wars 2 type system -- you now have more of a choice of where to go with a scaling mechanic. But when combined with 8.0's massive stat overhaul, things got murky.
Vol’Dun is the hardest and most tedious of the Horde Zones in Battle for Azeroth. It’s also probably the one that makes the most sense. Each sections of quests leads directly to the one you should do next. For all of these reasons it’s going to be best to get this one done early. Thanks to any legendary items you might have collected in Legion, you’ll be strongest in the leveling process before 115 and weaker after — because that’s when the legendary effects stop working.