Describe the solution you'd like
I'd like to have a system that rewards XP for completing it as a kind of mission and quest.
By using datapack, users can make PMMO like a MMORPG.

Minecraft rewards you when you complete advancements.
It gives Experience or Recipes for reward.
Add a PMMO system to this that player can get PMMO Experience by completing advancements.

Example File
return_to_sender_pmmo_example.txt

Why is this feature needed
Makes PMMO like MMORPG
Allows users to config easily by using datapack.

You can already award XP from advancements by using mcfunctions. Use the @s entity selector in your commands and it will work. This has been done by other pack devs, so I know this works.