Heroes of the Storm "expansion" - so what does that mean, really?

One of the most anticipated battlegrounds so far, The Battlefields of Eternity, hit the live servers after a week long stint on the Public Test Realm (PTR).  On June 30, the map's release kicked off what's being touted as Heroes of the Storm's first expansion, The Eternal Conflict, with new heroes, and new skins centered around the Diablo Franchise.

This sounds absolutely epic.  However, if there's something about it that doesn't quite make sense, don't worry.  You're not alone.  What's likely bothering you is that question in the back of your mind:  How do you make an expansion for a free to play game, anyway?

Kalypso Media rep hints at upcoming Dungeons 2 expansion or DLC

So I beat the campaign for Dungeons 2 the other day.  Overall, it was pretty fun, even if the narrator was just a little too chatty and the ending more than a little abrupt.  Like in many games, in Dungeons 2 you get a reasonable idea how far along you are by how much the campaign map has been filled out with completed missions.  However, the story introduced a couple of, what appeared to be, important plot threads, which it left unresolved, leaving you surprised when the credits rolled.

Gul'dan - how the warlock we all love to hate has cast a spell over the Warcraft fanbase

With the latest developments of Warlords of Draenor, Gul'dan has returned to the Warcraft universe as a major villain and for his first primary role in World of Warcraft.  Though I don't play WoW, I follow the story and after watching the cinematic from Patch 6.1 when he assumes control of the Iron Horde, I found myself really excited to see where Blizzard takes the plot surrounding one of my favorite villains.

But this begs the question: why is he so intriguing?  While important for lore reasons - being the one who set the orcs on their path to demonic corruption and invasion of Azeroth, etc. - as a character the first orcish warlock has very little depth.  His desires power simply for power's sake and will sacrifice anything and anyone to get it.  He's a textbook sociopath with exactly zero redeeming or even vaguely sympathetic aspects.

Despite all this, Gul'dan is widely considered one of the best villains in the franchise - why?  What makes the Betrayer of the Orcs such a compelling villain?