Pet battle challenge dungeons.
There’s also the Legion fishing artifact that is actually useful in DF.
There’s a ton of stuff from BfA from archaeology to islands that most people just never did or left unfinished.
Special mention to all the secret achievements like the Waist of Time, Lucid Nightmare, and the Hive Mind.
Glutton for Icy Punishment / Fiery Punishment & Shadowy Punishment (3 separate achievements in Azshara)
Fishing Diplomat; if you are feeling spunky go for Old Ironjaw and Old Crafty as well
The Cake is Not a Lie
Why? Because It’s Red
**This part is only sort of achievement related: during Legion I got very bored and worked to collect all the legendaries from Vanilla through Cata. You won’t get personal achievements for them all, however, provided your guild doesn’t already have it unlocked, you will get the guild achievement for having the legendaries.
Essence overload (rank 3 version of getting all rank 4s for the removed phenomenal cosmic power achievement)
(A) having a rank 3 essence on your account lets you buy that essence (for 2500 echoes of ny’alotha on any other character from MOTHER so you can do timewalking stuff quicker
maw walker title and all the other achievements that were part of veilstrider title (even though you can’t get it anymore)
(A) gives loads of mounts and chunks out alot of mog progress
the zg, naxx and soon to be scarlet monastery (10.1.7) quests to unlock ability to farm for removed mogs.
I had a ton of fun doing the secret fish and where to find them. You get to keep the goggles too, which are just as fun as the hyper compressed ocean. I do wish they’d update the fishies in it for each expansion.
Exalted Argent Champion of the Horde: Chasing Crusader title for my paladin was pretty fun. A month of logging off at Argent tournament grounds, good music, nice dailies, just pure serenity and peace of mind.
Now I Am the Master: I’m teaching Nomi how to cook in Pandaria so he can grow up to be a jerk in Legion was pretty fun. Plus I got apron, frying pan, rolling pin transmog from it to go with my Chef title, so time well spent.
Brawler for Azeroth: Getting Brawler title for my orc warrior named Orcybalboa was pretty fun.
Loremaster: Most relaxing achievement there is. Good times, good times indeed.
I missed some of the expansions from the past and this allows me to live some of that content, kind of…since I 1 shot everything. But, I’ve always enjoyed lore stuff but I normally don’t give it the time of day because I’d prefer to be doing m+ or raid.
I may start on it again but it is one of the achievements that I’ve always wanted to get.