It might be nice to include gear drops. Could use transmog for completion status. (Though only for gear the current character is allowed to equip.)

This would be a lot more loot, so we'd presumably want to datamine it out. We'd need to decide what loot threshold qualified (blues or above, 25%+ chance? Anything more niche could be added by hand...)

Step towards the former comment: we now have quests associated with items. Adding achievements is just a matter of mining the data.

This is now possible, but mostly only has the existing mount/toy/pet stuff included. I filled out mechagon and timeless isle.

Mechagon / Timeless do expose a new wrinkle that I haven't addressed yet: they have loot that counts towards an achievement. We could note that, and track it towards "completion" for #53.