If you are just coming out of Level 110, you will probably have a number of different Legendary items to choose from. Your Legendaries' special effects will stop working at Level 116, but until then they can be a very powerful addition to help you level up quickly. These are our Legendary recommendations for leveling as a Fire Mage in Battle for Azeroth:

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.

Island expeditions are very easy to complete while leveling, particularly before hitting Level 116 when your legendaries are still in effect and your secondary stats have not been squished by leveling scaling yet. Island expeditions can be completed in 5 to 10 minutes relatively reliably while also acquiring some of the pets, mounts, and Bind-on-Equip gear available. There is also one weekly quest to do one island expedition, so for slowly leveling alternative characters, be sure to complete at least one island expedition each week.

Heirlooms. At the bare minimum you make sure all the armor pieces upgraded to 110, since this is the lion's share of your experience increase. Neck and Ring upgrades are secondary, but still important due to being able to enchant them. Trinkets can't be enchanted, but you'll want them at max as well, if only to save time by not having to worry about replacing them as you level.
/way commands like /way Shadowmoon Valley:Draenor 50.0 31.2 Ashlei don't work correctly. According to the way TomTom works, this waypoint should be created in Draenor's Shadowmoon Valley. Instead, it will be created in Outland's Shadowmoon Valley. So it seems there is currently no way to distinguish between zones that occur more than once with the same name, such as Shadowmoon Valley or Nagrand.