Pure speculation. Nobody has a freaking clue.

Early on at the initial release, Blizzard stated that their original storyline takes them out to level 90, but they refused to release any details of what that storyline entailed. They did "reserve the option" to extend the game even further, but I think that all depended on the success and money in doing so. Somehow I don't think they'd have any issue with trying to extend this game as long as they can. Granted, this was about 4 years ago, and their business strategy could have drastically changed. They did mention that they intended to have a final "end game" boss at the end of it all. Considering the Lich King has been a puppet of several different masters, I can hardly believe that he's the end to any storyline.

I think your friend may have heard those kinds of details that Blizzard had a predesignated "end" story for the game and may be misinterpreting that this was the end.

I think an Emerald Dream expansion would be easy to pull off and would make for a perfect next extension of the storyline. We now have the ability to face off against 2 Old Gods that have corrupted the world, but there's a minimum of 3 others (that we know of, total number of Old Gods is unknown) that may still be bound in the dream world. I doubt any more would be revealed before the Lich King instance(s) is opened. The emerald dragonflight is still battling against them in that realm, and emerald leader is slowly going insane from it (re: Dragonblight emerald sanctuary story questline). Throw in the fact that there's like 4 portals pre-placed around Azeroth surrounded by green dragonkin that is serving no purpose except to slaughter lowbies that get too curious, I think Blizzard has already made that a very strong possibility. The Old Gods also corrupted Deathwing (if I recall correctly), so he could also re-emerge as one of their puppets, which common thought is he is still alive (and possibly leading the mysterious Infinite Dragonflight).

Anyways, you could go on and on and on. As falsfire mentioned, a South Seas expansion is another possibility.