If you just care about unlocking the ability to fly in the new expansion, and you should, the site’s Pathfinder tool is a must-bookmark. Its Character Planner is also incredibly useful for getting a sense of what you could (or should) do next, based on your recent activity, the gear you’re wearing (and would like to upgrade), achivements you’re close to unlocking, et cetera. If you want a simpler guide, the site also has a great list of all the activities you should be doing each day and week in Battle for Azeroth.
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.
This expansion suffers from the Warlords of Draenor (WoD) effect, initially, players will think this is a great expansion but upon investing some time into the game they'll realize this game is a disaster. The pre-patch should have been our first warning with how poorly designed and executed that fiasco was, but beyond that, let's look at what the expansion itself offers:
At 96 the standard wisdom is to immediately drop what you're doing and proceed to Spires of Arak and do the quests to unlock the Inn, granting a further 20% experience bonus while in that zone. You'll want to do that eventually anyway, but I'm not sure it outweighs the quest experience from Gorgrond. However, if you decided to go to Spires immediately at 96, restrict yourself only to bonus objectives and again leave Treasures until you're 100+. Make certain that when you being the quests to unlock the inn, you speak with the quest NPC to also learn Archaeology, since it's required to collect several of the treasures in Spires.
World of Warcraft: Battle for Azeroth focuses on the rivalry between the two most important factions of Azeroth, the Alliance and the Horde. The expansion adds two new continents (Zandalar and Kul TIras), divided into islands, where you can find new dungeons and raids, quests and previously unseen enemies, as well as six new playable races. Once again, the maximum experience level of the player characters increases (from 110 to 120). Battle for Azeroth also introduces two new game modes. In the first one, groups of three characters explore chosen maps and fight randomly assigned monsters and threats. In the other mode, groups of twenty characters each try to take over locations in strategic places for their factions.
Gold for riding training. Travel time is a SIGNIFICANT time sink while using quests to level. You don't want to wait to get it. In fact, while questing, the second you ding 40/60/70/80 you'll want to hearth and train it IMMEDIATELY! 10304 gold is the base cost, which will be modified by reputation. Also, make sure you're in a guild with the 10% riding speed bonus unless you're a class that already gets a mount-speed buff.
Anyway both got to 110 at around 50h played. I messed up with my Pal by going to Legion once I hit 100 instead of finishing treasures and bonuses in WoD. All in all, if I picked the best zones and was only trying to level as fast as I could, I would probably could have done it in roughly 40h. Note that it would have a been A LOT faster with a Monk as both Ret and Rogues don't really have a decent AoE until 40+ (45 for Ret and 63 for Rogue I believe). The daily also helps a lot. I know that leveling my Monk before the pre-patch was insanely fast. Might be slower with the changes to FoF, however.