• For killing / interacting / looting something which is not directly a quest objective.
  e.g. The quest Marla's Last wish (6395): You kill Samuel Fipps (1919) and then take the remains to the grave.
• Syntax could be [NPC1919 Samuel Fipps] / [ITEM123 xyz] / [OBJ456 abc]
• It woud need to be specified what exactly needs to be done for cases where it is not kill / interact / loot? E.g. interacting with an npc rather then killing it. Or only showing a tooltip without having to do anything in particular.
• Coordinates could then be added automatically.

For items this has been implemented with [CI] back in addon version 1.045

[TAR] also creates coordinates now.
so left here is:

• a version of TAR which completes on kill / interact
• objects

(optionally) specifying an amount would also be necessary (kill x mobs / loot x items).

for items it would need to be checked whether / how many are already in the inventory

Progress (how many mobs were killed / items were obtained) would need to be stored in saved variables since it wont be detectable automatically. When items are destroyed / sold the step should (probably?) not become active again.

Example from _hicken#6446: horde quest: “Demon seed” requires you to pick a flawed power crystal off a table. Then use that item on an alter.
https://classic.wowhead.com/item=4986/flawed-power-stone
Wowhead
Flawed Power Stone
Flawed Power Stone is a quest item. It is a quest reward from Flawed Power Stone. In the Items category. Added in Classic World of Warcraft.

So like;
Loot [QI4986] off table next Derby Derp
Do [Q924]