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
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.
Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.
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.
Heirlooms can certainly be used with our guides! You may find yourself getting ahead of the guide. If you start doing gray quests, just skip ahead to a later zone. The guide is just that - a *GUIDE* not a rule, as many people seem to think. Feel free to skip parts! Watch out for quest chains, though, especially in the last part of the 50-60 section. Hopefully we'll have some awesome features in our new addon that will make it easier to skip around!
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.