Picking up herbalism and mining is the best professions to have while leveling. Not only will you earn gold as you level, but you will also get extra XP. At level 20 you will get over 500 XP for picking up a ore or a herb while killing a mob only gives a little over 100 XP. Don’t go out of your way to pick herbs if you are trying to level quickly, but pick the ones that are on your way. Leveling from 110 to 120 can be done pretty quickly just by doing this. There are a few items that will help you if you decide to use professions while leveling:
New to Patch 8.1 is a new leveling flask called the Draught of Ten Lands which increases all primary and secondary stats by 20 and experience gained by 10%. This effect lasts for one hour and persists through death, similar to other battle flasks. This item can be purchased using a new Patch 8.1 currency called 7th Legion Service Medal or Honorbound Service Medal which can be earned doing the new incursions and Warfront activities. Simple earn these medals on your main character, purchase the flasks, and send them to your other characters. The cost is 10 of the medals.
The prepatch for Legion, for example, offered players in-game demon invasions throughout the world of Azeroth, presenting opportunities for new appearances, feats of strength, a pet, a toy, gear that provided a catch up in power for those with alternate characters or who hadn't played in a while and the opportunity to infect other players via an in-game mechanic.
Most of the time you spend leveling will involve questing and slaying monsters, but there’s also a lot of travel. Travel earns you little experience (you do gain a bit for discovering new areas), so it’s wise to keep travel to a minimum. The new level scaling system, which scales zones to your level within a preset range, helps with that. You can choose what zones you want to experience and stick with them until you finish their quests.
The recruit also grants the recruiter levels, with the latter gaining one level for every two levels the recruit gains. There are a lot of rules and restrictions to that, however. It all basically boils down to leveling alternate characters together. The recruiter can even earn epic mounts and battle pets if you stick with the game for a couple months.
When an allied race has been unlocked for a player, new characters from that race will start at level 20. If a player levels an Allied race from 20 to level 110 without using character boosts, they will be rewarded with a unique "Heritage Armor" for transmogrification that reflects the unique lore behind the race but is limited for use to that race. Doing the quests to unlock the allied races will also unlock the races' unique mounts for use by the characters of their faction. The void elves, Lightforged draenei, Highmountain tauren, and Nightborne, as races tied to the storyline for Legion, were made available with the preorder on January 30, 2018. The Dark Iron dwarves and the Mag'har orcs are made available upon completing the "War Campaign" in Kul Tiras and Zandalar and reaching Exalted with one faction's War Campaign reputation; the Kul Tiran humans and Zandalari will be made available later in the expansion. Game director Ion Hazzikostas also indicated that additional Allied Races may be available in the future.
Warfronts are larger, 20-man challenges that take inspiration from the Warcraft strategy games. Players are deployed on the Warfront to collect resources, destroy enemy troops, construct buildings, and eventually launch an assault to take over the enemy base. These battles take 20 to 40 minutes to complete and, unlike Island Expeditions, don’t offer a PvP mode.
Each of those modes existed long before this latest expansion; raiding is a basic concept in MMOs, and Mythic+ was added in World of Warcraft’s last expansion. The problems with Battle for Azeroth’s endgame come from the systems it adds, which most often artificially gate progress where most of the fun can be found. If raids and Mythic+ are the carrot, we’re about to talk about the stick.
Dugi’s leveling guide is the leveling addon I’ve always used to level as fast as possible. It will show you where to go and what to do in-game, speeding up the leveling process. This is optional though, as the game already does a pretty good job at showing you where to go at all times. The addon does, however, come with a couple of other nice features and perks.
Warlocks have received new spell effects; all the other classes received new effects in Legion. Male orcs are now able to toggle between the initial hunched posture and standing upright by using a barbershop, but the other races are not able to toggle between two postures. There are new druid forms for the allied races, i.e. a Zandalari troll forms are all dinosaur-based, while the Kul Tiran humans take forms made of wood and bone as their teachings come from the Drust people who populated the land before them rather than the Cenarion Circle.
I do not recommend going for TBC, although I remember Hellfire peninsula to be decent, everything else is rather slow. Instead go for WotLK, Borean Thundra gets you levelled very fast, and then you can head straight for Icecrown at 67, which is also insanely fast, although admittedly has a few elite quests. You can go to Sholazar Basin alternatively. 2 zones should get you 60-80 no problem.