Nice idea.
I'm not a mapper, but as a player I was thinking at something like this, but in a different way :
Previously, I select (or add in a locked collection) the Y maps that i want to track.
Then, I will to be notified when player X start to play one of the Y maps. (specific chat for theses notifications)
Or / And mixing it with the filter "Map" idea :
When selected, we will find all the players who are currently playing the Y maps.
Problems :
- too much requests / charge for the server?
- can generate lots of flood with notifications with an high amount of Y maps.
How about limitate the maximum Y maps to track at the same time? 3? 5?
I may be a little off topic here, but the filter Map feature can be useful.