Hi, I just wanted to share this idea with you.
Well, the title is self explanatory. Have an option to create NPCs for customizable quests in game.

QUEST CREATOR SCREEN (Step 1)

The quest creator screen would be an option maybe in the Sims' Worlds screen. This quest creator would have 2 options: Create a NPC, Delete a NPC.
You would be able to use Sims from the Other Sims list only. Once you create the NPC, you would have to name it.

QUEST CREATOR MENU (Step 2)

Once you select the Create a NPC. You will have the following options:

Select Sim: Select the sim from the other sims list.
Select location: Select the lot where you would like the sims to be hanging out.
Select emotion: Select the emotion of the NPC.

QUEST CREATOR MENU (Step 3)

Once you have your Sim selected and the location, you would be able to add triggers, that would be classified as INTERACTION or RECEIVE.

Interaction triggers would be actions that will trigger an answer from the sim. You would be able to select the interaction.
Receive triggers would be actions that will trigger an answer from the sim after giving the sim something.

Next to each interaction, you would be able to select the reaction. The reactions would be classified as MESSAGE, GIVE, ACTION, BUFF.

Message reaction would be a customisable message from the sim after interacting.
Give reaction would have the sim give you an item after interacting.
Action reaction would have the sim performing an interaction with yours.
Buff reaction would have to get a buff from the sim.

Every NPC that you would create would be able to have a limit of 5 triggers and 5 reactions. And the NPC quest creator screen would be something like this.

- Select Sim - - Select Location - - Select Mood -

- Select Trigger - - Select Interaction/Item - - Select Reaction - - Select Interaction/item/buff/message -
- Select Trigger - - Select Interaction/Item - - Select Reaction - - Select Interaction/item/buff/message -
- Select Trigger - - Select Interaction/Item - - Select Reaction - - Select Interaction/item/buff/message -
- Select Trigger - - Select Interaction/Item - - Select Reaction - - Select Interaction/item/buff/message -
- Select Trigger - - Select Interaction/Item - - Select Reaction - - Select Interaction/item/buff/message -

You would also have an option to see the active NPCs you've created and would have an option to delete the NPC, this would probably delete the NPC from your Sims screen.

FINAL EXPLANATION

So, explained everything above I will leave you an example.

I created a NPC of Bob Pancakes hanging out in a library lot in Willow Creek. I named him Stressed and chose a tense mood. So in-game would appear as "Stressed Bob Pancakes".
I gave him the following interactions: Ask how is your day and give.

I bring my sims to interact with him and select give, this will open my items in inventory to give him and select a random item, this won't trigger anything because I won't give him the item the needs. Then I select the option "ask how is your day" and this will trigger a message "It's going awful, I lost my favourite book "Motivational Book". Now I know he needs this item, so it's time for me to find it and bring it to him. Once I have it, I select the option give and select the book. This will trigger a message "Thank you very much for helping me! Here have this as a reward", followed by a hug interaction and a skill book.

Well, that's basically it. What do you think? Maybe would come good with a fantasy expansion pack that includes quests.