A configurable view distance beyond which all players will be vanished (unless you have a certain permission node)

Would be nice!

Sure, there's a "player-tracer" client mod going around to track players with tracers at extended distances (so this would allow for those players to be hidden) until a certain distance

Sounds interesting but can't seem to figure out what it would be used for... Can you share a scenario

Then the plugin should maybe also read the view-distance in the server configuration and set the player view distance to something fitting by default.

Would be nice too if there was a (configurable) block behind which players are hidden. So in a Hardcore PvP servers players could surround their underground or secret rooms with for example Obsidian and they can not be seen while behind it. Not seen as in no nameplates, since hacked clients could still see the cave.

As for what codename-B suggests, I think this would be a really useful feature. I would definitely use it on my server.

Closing 4.0 issues, I'd rather not have scattered planning for the future which may or may not come to pass

I assume the view-distance in server.properties doesn't quite fit your needs?
This would certainly be an interesting option, whether it be in this plugin or in its own.

rakiru, players are loaded even behind the view-distance as far as I know.

Ah, I was lead to believe that they're not, which would make sense as it would require less bandwidth.

I'll be playing with this in the sneak branch.