[Update]: Blizzard has now added a 100% experience buff for all players until April 20th, so if you ever wanted to level an alt, this would be a pretty good time to do so.

Despite Blizzard doing their best to promote alt characters through Allied Races and reduced experience requirements, Battle For Azeroth still remains rather unfriendly to alts. As soon as you reach level 120 with a brand new character, you essentially have to do a dozen different things in order to catch up, all of which require a considerable amount of time. Nowhere is this more apparent than with Essences which provide a sizable boost to power, yet often require you to do outdated content for weeks on end.

As such, I am glad to say that Blizzard has now introduced an easier way to acquire Rank 3 Essences on alts. Simply complete any of a variety of endgame activities and you will receive Echoes of Ny’alotha, which you can then trade with MOTHER in the Heart Chamber for an unlocked Rank 3 Essence of your choice. It'll take you 500 Echoes to acquire each Essence, but don't worry about this being too grindy as the amount you'll gain by simply playing the game seems to be quite high:

• Black Empire Assault – 125 Echoes of Ny’alotha
• Aqir/Amathet/Mantid/Mogu Assault – 75 Echoes of Ny’alotha
• Horrific Visions – 50 Echoes of Ny’alotha, plus 25 per additional objective completed (i.e. 150 Echoes for a full clear)
• Emissary Quest – 50 Echoes of Ny’alotha
• Heroic Darkshore Warfront – 75 Echoes of Ny’alotha
• Normal/Heroic/Mythic Ny’alotha raid bosses – 15-25 Echoes of Ny’alotha each, once per week per boss
• Ny’alotha Raid Finder wings – 25 Echoes of Ny’alotha
• Random Battleground wins – 15-25 Echoes of Ny’alotha
• Rated Arena wins – 10-25 Echoes of Ny’alotha baseline, scaling up with PvP rank
• Rated Battleground wins – 35 Echoes of Ny’alotha baseline, scaling up with PvP rank
• Weekly PvP Cache – 175 to 300 Echoes of Ny’alotha, based on PvP rank
• Mythic Keystone dungeon end-of-run chest – 3 Echoes of Ny’alotha per Keystone Level (i.e. 30 Echoes of Ny’alotha for a Mythic 10)
• Weekly Mythic Keystone Cache – 20 Echoes of Ny’alotha per highest Keystone Level

"The general goal here is that doing Assaults and associated Horrific Visions should provide enough to get at least one Rank 3 Essence of your choice per week," reads the developer update. "Doing other content you like (PvP, dungeons, or raids) will increase that significantly, depending on the level of content you’re doing. Of course, Essences can still be earned normally via their associated activities, so if you want to get your Essence of the Focusing Iris directly via Mythic Keystone dungeons and then buy others from MOTHER, that’s now an option."

"More recently, as we continued to revisit the subject, especially in light of ongoing Shadowlands design, we came to see subsequent playthroughs of the same content for the same player differently. While we still firmly believe that each character should have a progression arc of power gains, the player behind those characters understandably may not want to repeat the exact same arc multiple times when they feel they have exhausted the variety or rewards an activity might hold. So if you’ve thoroughly worked with Nazjatar followers to earn your Rank 3 Memory of Lucid Dreams, or delved into Battlegrounds and Island Expeditions enough to earn Rank 3 Blood of the Enemy, we understand why you might not want to feel required to repeat that experience."

You can read a little bit more about Blizzard's reasoning behind these changes, as well as follow any future developer updates, over at the official forums.