Bags: This SHOULD be a no-brainer. The cheap-o version are Netherweave bags, but if you have the gold I recommend you get the largest bags you can afford, since it means you won't have to stop to clean them out as often. In fact, if you have an excess of gold, it's probably not even worth your time to stop and loot mobs that aren't quest objectives. This seems like a small thing, but over the course of 100+ levels, all those clicks to loot add up.
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.
Just like player item levels, the game's professions have become pretty bloated over time. Blizzard has made some updates to the leveling process before but Battle for Azeroth features a skill squish that should smooth things out considerably. Instead of having to level hundreds of points before you can start crafting Battle for Azeroth items, each expansion will have its own separate skill tier. In other words, you can start crafting Battle for Azeroth content right away, while the gear and items from other expansions will each feature their own separate counters. Your first Battle for Azeroth item will put you at 1/150 on the counter for the new expansion, no more grinding needed.
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":" - Any time a step ends in a ":" instead of a "." means that the next step is part of the current step you are on. This generally means the next step should be done while working on the current step. This means every "start working on" or "continue working on" will end in a ":". But this is used with other occasions as well, so keep this in mind.
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.
REDISCOVER AZEROTH AS NEW ALLIED RACES – Take a new form for your adventures as new playable Allied Races. Embark on a quest to earn the favor of the Lightforged draenei, Highmountain tauren, Nightborne, Void elves, Dark Iron dwarves, and Zandalari trolls to unlock each race and add their strength to your faction. Create a new character and complete the full leveling experience to earn a distinctive Heritage Armor set.
I made some route changes to the Horde levels 43-44 sections. I have swapped 44 Dustwallow Marsh with 44 Desolace (the entire sections). This allowed me to do Deadmire (at lvl 43 instead of 38) and then go stop at TB to turn in Deadmire + The Black Shield at the same time, then fly quickly to do the Desolace stuff. Doing 44 Desolace is now mandatory because I think its faster with the new routes. This will also make the level 53 grind much shorter. I think these were great changes.
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.