At 98 you have the option to drop what you're doing and return to your capitol city to start the quest for your Legion Artifact weapon. However, I do not believe this is necessary to do right away unless you're struggling with kill speed in your current gear. I would recommend waiting for a natural pause, such as hearthing to repair and clear your bags. From 100 to 107 you won't even really fight mobs that often; only just enough to clear the way to any treasure you need to pick up.
Another popular leveling guide is called “Booster”, which is also available for all races of Horde and Alliance. Similar to Zygor, Booster is an addon that you view inside the game, and it tells you exactly where to go and which quests to do every step of the way to level 120. The leveling path was created by professional gamers, and optimized to make your leveling as fast as possible. Booster also comes with free lifetime upgrades and updates, so you literally just have to pay once and use them forever, which is awesome. It’s created by a company called Dynasty Addons, that has been around for almost 10 years, and their guides and addons have always been updated in a timely manner for every expansion pack. It’s easy to see why this leveling guide is so popular.
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