For collecting artifact power to level up your Heart of Azeroth necklace, island expeditions are unbeatable. There’s a weekly Azerite goal that you need to meet through the expeditions, and when you do, you’ll be awarded 2500 artifact power. There’s a small chance that you’ll get weapons from completing expeditions too, but they’re not a reliable source of gear.
Once you reach an Alchemy skill of 475, you may begin a quest to learn one of three specializations: Potions,Elixirs or Transmutations. Being specialized gives you a chance to create an extra 1 to 4 (for a total 5) items while crafting in the field you have selected. For example, a Master of Potions may be able to craft more than one [Super Mana Potion] for the same material cost as one.
The tail end of Legion saw the introduction of Allied Races, which introduced two new races for each side to play as. Battle for Azeroth introduces more, with Dark Iron Dwarves and brown-skinned Orcs included in the upcoming choices. Blizzard hasn't confirmed much about what other kinds of races might show up, but there's plenty of datamined info out there for those who want to take a look. Allied Races start at level 20 after a short scenario and players can earn a special set of cosmetic armor if they level the character all the way to 1110 without buying a level boost.
If you love running old content for transmog items, you'll love the new legacy loot system. Whenever a player enters a dungeon or raid where they are 10 or more levels above the content, the dungeon or raid bosses drop an amount of loot equivalent to what would have dropped for a full group when the content was current. That means players could earn loot for five people in dungeons and loot for 20 people after soloing a raid. The legacy loot system also drops other gear types, like cloth dropping for a plate wearer. It's now much easier to get the transmog gear you want, and you can also collect gear for your alts on the same run.
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.
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.