Just recently, Greyseer (the author behind Lorecrafted), finished up a three part speculation series on what he would like to see come out of a Maelstrom expansion. Reading over it, you realize just how many different forces could be at play in the expansion. I don't think everything will play out the way that he thinks they'll play out, but that just lends more weight to just how much could happen in the Maelstrom.
There are plenty of people that think Warcraft dies with Arthas, but when you look at the game world beyond the events of Warcraft III, Arthas is actually relatively small character. He's not even the most dangerous thing Azeroth has seen. The threats that lie within the Maelstrom have proved themselves to be far more destructive... as in, caused Azeroth to sunder into the various pieces that make up the world today. Let's see the Lich King do that.
The Maelstrom opens up opportunities for not only new stories and characters, but the return of some old ones. The Emerald Dream will most definitely play a role, which means Malfurion, Ysera, and that whole crew. Maybe we'll finally get to see Fandral Staghelm's true face: Is he good, evil, or something in between? Queen Azshara will definitely make an appearance, as well as Neptulon. Considering the Maelstrom is home to the Tomb of Sargeras and the Well of Eternity, which was the portal through which Sargeras would enter our world, we'll probably see a significant amount of Burning Legion involvement, too.
Greyseer also suggests we might see the return of Maiev Shadowsong in a fairly significant role, which would please me to no end. She's been one of my favorite Warcraft personas since she made her first appearance in Warcraft III. The death of Illidan is a fairly fitting end to her story, but I still think she's a character that can be taken much farther. She's a person who literally had everything taken away from her by her own people. That's harsh! I'd love to see what she does with her life now that Illidan has been dealt with. Does she fight the good fight against the Legion and whatever else the Maelstrom throws at us? Does she reject the Night Elves and go and do her own thing as a neutral character/faction?
Greyseer and I probably wouldn't agree on everything we expect will happen in the Maelstrom, but reading what he writes about the topics certainly makes you think, and if his ideas get people to start speculating on their own, he's done a good job. Go give his posts a read, he lays them out in the format of a progressing expansion: the launch, patch 4.1 and patch 4.2, then patch 4.3 and patch 4.4.