Not sure if something like this exists, but in battlegrounds you can't put raid target icons on enemy players (not sure why). As I've been playing around with nameplates I've found that I can add icons all day long to nameplates on my own screen. If everyone had the same addon, you could tell their addon to display the icon on the same nameplate as yours.
This would allow you to mark someone and everyone around would focus fire on a single player, taking them out faster. Of course you could just say "everyone kill hardtopronouncename", but people are more likely to attack whatever has a skull over its head.
Also, not sure if this would be trying to circumvent something that Blizzard doesn't want you to do.
Not sure if something like this exists, but in battlegrounds you can't put raid target icons on enemy players (not sure why).
For the same reason why you can't do it in arenas either. Enemies that go out of line of sight, or carrying the flag and hiding in WSG, aren't supposed to be seen by icons floating over their head from a mile away.