Yeah, completeing Neeru Fireblade gives notifications about Felwood unlocked, Lord Banehollow — about Winterspring, and Ahune the Frostlord — the rest of the areas (all tested on Heroic). Some of the latter notifications are fun if you haven’t seen them, but it gets a bit old.
There is no such Mercenary in the game (not even as a special visitor only) at the moment. 