Something I've only seen in a few games such as Warhammer Online and a few others (that I can't remember at the moment) I found very neat, convenient and immersive is kill quests taking into account your kill count of the mob from before you accepted the quest.
It's super nice to pick up a quest and be able to turn it in immediately because while you were out exploring the wilderness you happened to already cull the population the npc is asking you to cull. Not to mention it makes some sense immersion-wise.
Edit: Not to mention it helps foster exploration, as people will be more open to exploring and killing random mobs that they usually wouldn't if they feel it's not pointless trash mobs.
Example in my next comment below
Edit2: To clarify, not all quests have to be retroactive, just the ones that it makes sense for them to be. And them being retroactive doesn't mean you can't accept the quest and do it normally if you haven't completed any of the objectives yet. (I think that's where
@Noanni got confused)
(Also edited title to be more accurate)
Edit3: A perfect example of the type of quest that could be Retroactive is
the Hogger quest in vanilla/classic WoW. During normal questing in Elwynn Forest, you might come across this named NPC and think, "Oh, a named NPC, he must be important later".
Maybe you then kill him just because he's there, but then later get the quest and have to kill him again.
Maybe you don't because you figure or know you'll have to come kill him later.
- In the first case, why then should you have to kill him again later just because "suddenly there's a quest to kill him". You killed him... he should be considered slain, right?
- In the second case, why should you even have to feel that way. Why should you feel discouraged (even in such a minor way) about killing something if you felt like it.
Both of those issues can be solved by making the quest retroactive. Anyone in this thread complaining about random enemies not making sense being retroactive, what about named npcs that are quest objectives? If you killed him before you were able to get the quest, the quest considers him already slain. If you haven't yet, the quest prompts you to go kill him.
I will say, another alternative to this is better quest writing, and having quest NPC's only spawn if you have the quest and things like that, but for non-named NPC quests retroactivity, when the quest writing allows for it, would fit nicely.