Once again, as soon as you hit 70 - I recommend to go and learn faster flying. This speeds up your questing A LOT, and is worth the time and gold. It will cost 4500 if you go to the capitol, or 5000 if you prefer Shattrath/Dalaran, because you will not have reputation with them at that point. This price will sadly be above what a totally new character can afford, if you don't have any Main. But you should go and buy flying as soon as you have enough money.
This whole affair has caused myriad arguments with the community, with some old-timers noting that the time-to-level increase is a joke compared to Vanilla, and newcomers sharing their point of view that World of Warcraft is slower than a lot of other games on the market. I can certainly understand both mentalities, but given that the leveling system is sort of a token process now and is already frustrating for a lot of players -- seeing it shaken up this much can't be good for the game.
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.
A: The gold value of a Token will be determined dynamically based on supply and demand. When you put a Token up for sale, you’ll be quoted the amount of gold you’ll receive upon a successful sale. If you then decide to place the Token up for sale, that amount is locked in, and the gold will be sent to your mailbox after another player purchases your Token.
Excess Potion of Accelerated Learning: Acquired in your Garrison (part of the Warlords of Draenor expansion), this gives you a 20-percent bonus to experience from killing monsters and finishing quests. It lasts one hour, but you can buy it multiple times and use it back-to-back. Alliance players buy it from Sergeant Crowler, horde players buy it from Sergeant Grimjaw. Does not work above level 99.
Lastly: The zone order I recommend for Horde is Vol’dun- Zuldazar- Nazmir. Vol’dun is definitely the most challenging of the bunch and having the use of your Legion gear and leggos will smooth this zone out. Nazmir is the easiest of the bunch which is nice when things start to get a little grindy past 116. For Alliance, I recommend Drustvar- TS- Stormsong. Same logic as the Horde side, in my opinion Drustvar is the hardest and Stormsong is the easiest for when it gets grindy. I would recommend you waiting to do any of the War Campaign until you are capped because it just takes you out of the way. I also recommend not killing a single rare mob or picking up a single treasure because in my experience it is just not even remotely worth it. The gear scales up on the rare mobs so its smarter to keep them till 120 anyways. After you are done leveling, I do recommend taking the time to do all the quest at the Faction Outpost in your enemy territory. They do provide rep for the Honorbound which helps. Most of them are quite easy. If you do not level in Warmode, you will usually hit 114 from your first zone, 117 from your second, and cap 120 about 2/3 in your final zone.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide