What functionality is missing? What is unsatisfying with the current situation?

I'll start by saying maps in World Anvil are amazing, I used them as a base of operation for my game creating markers that link from my world map to city/towns/dungeons/etc to houses/hideouts/etc and back again with labels to relevant articles on every map. This is amazing for me as a DM but when it comes to my players I don't want to show all my cards early by letting them see what's ahead. I have recently started running Curse of Strahd and want to give my players the ability to see the places in Barovia they have been while keeping the rest of the map obscured.    The goal is to keep a record of what players are aware of and have it displayed for them, (players are well known for their ability to perfectly remember where they have been and why), while keeping them in suspense by hiding future locations from them.   I am proposing a feature that gives the ability to shroud maps in fog that can erased away as required, revealing the map to the players. The shroud could exist as an entity over the map, or be in a new layer that sits in the foreground. It could be with a solid that could be erased as needed or a series of segments that can be turned off and on as required. Perhaps the segments could be linked to articles to automatically be revealed when that article is shared.

How does this feature request address the current situation?

The ability to shroud a map in fog is a powerful tool using in RPG's to keep players aware of only what they can see. This tool gives story tellers and game masters more control over the narrative by making only desired and known information visible to the players. 

What are other uses for this feature request?

Reasons for this feature to exist:

• Prevent players from being overwhelmed by the size of your world, allowing you to slowly introduce new areas as they explore the world and interact with its people,
• keeping player attention away from areas that are secret or have yet to be contracted by the game master (lets be real, we only have so much free time and our worlds are grand),
• keeping the element of surprise, when players start to explore new areas they will be more engaged as everything they will see is new to them 
• tracking the characters knowledge of the world/map, by removing fog from areas they have been, have been told about or have prior knowledge of
• rewarding interacting with the world and its people to find new locations and learn about them
• subtle guidance as they know where they have been

Why stop at fog:

• if you can change its coloured to resemble water to hide undiscovered islands, reefs, whirlpools etc,
• or sand to cover up lost cities, caravans and more in large deserts
• an inky black covering that could hide miscellaneous objects floating in space

Addition future additions to the map shroud: 

• the fog could be linked to an article, when the party discovers a new location you give them access to the article and the map shroud covering this new area vanishes.