While there is certainly some potential for entirely new storylines, I'm more interested in the places that have only been hinted at, haven't been seen in a while, or might not exist at all. Argus is a prime example (Draenei try to retake their home, goblins accidentally make a wormhole while trying to boil an egg, Bronze Dragonflight flips out and clones the Dark Portal, etc), or Kul Tiras, maybe an undiscovered fragment of Draenor. Perhaps the goblins try to recolonise the ruins of Kezan.
My favourite candidate for this was formerly the home of the worgen; not the pseudo-werewolf Gilneans, but the creatures summoned by Arugal and his ilk. There was much speculation that the enemy they fought, who wielded "green fire" was another branch of the Burning Legion. Was disappointed to find that this wasn't the case.
It's also true that there is potential for entirely new characters to shine, Sunwalker Dezco being perhaps the best example. But the game has a bad habit of making new characters with a lot of potential and then discarding them. I think here of Forward Commander Onslaught, Voren'thal*, Vanessa VanCleef, "The Ebon Watcher," Gallywix...
*And when is he going to question just who sent him that vision?
There's even occasions when an older character would serve perfectly well in a role, yet instead you get a new entry from the random name generator. For example, where did Valithria go? Why couldn't she have served as one of the dragons in Dragon Soul? Or even, in a particularly tragic turn of events, have been the dragon who fell to corruption to rise as Alysrazor? Returning characters establish a sense of narrative and continuity, yet there are so very few of them, and many persist only long enough to become a boss (Zaela) or trapped in a dungeon forever (Deathstalker Belmont). Brann is one to break out, but even he only crops up in plot fragments, holding no connected narrative of his own.
No, what I want to see is a long-term development of a series of characters.
It's well known that WoW doesn't have the time or resources to devote to all of its stars, that's why Thrall keeps popping back up. Despite his status as a "tired old character" (I forget which dev said that), he has name and character recognition, qualities that are time consuming and resource-expensive to grant. It would be weird to have his role played by, for example, Baine, despite broad similarities in philosophy.
I'll try to keep it brief, but what about elevating some old, backstage characters to playing a more central, repeating role? Nazgrim was a good example, but what about Faranell, still dealing with the repercussions of Putress' betrayal? Mia Greymane, becoming a political player in her own right, outside the influence of her husband. Nathanos Blightcaller, playing Varimathras' role better than the old demon ever could (and for that matter, we know that Varimathras, Balnazzar, Detheroc and Mal'Ganis are still wandering around out there, when are they going to show up again?).
On the subject of redemption, I do think that Warcraft desperately needs a redemption story, we've had far too many "oh they're evil now" stories and been forced to slay people who were prime candidates for forgiveness (Illidan, Kael'thas, though I wouldn't go as far as to say Arthas. And speaking of which, if GARROSH of all people gets this opportunity then someone messed up, if there's one character who doesn't have even a shadow of an excuse it's him). I'm particularly interested in one example:
Sylvanas. As many have said, she's grown entirely too close to the "become what you hate in order to destroy it" model, the phrase "Lich Queen" is being bandied about discomfortingly readily. There's even still the possibility that she had a hand in the events of the Wrathgate. This has to be reversed before she becomes yet another dead/deposed racial leader, written into a corner before finally being killed off because nobody could think of a way out.
Here's your way out: her sisters. It's been mentioned that Alleria and Vereesa would be most likely to slay Sylvanas as a mercy to her, but why wouldn't they try to help her? I'm not saying that Sylvanas must rediscover her humanity and become all penitent, that would be boring as well as cheap. She has a role to play, and that role is to be the foil to Thrall, Vol'Jin, Lor'themar and Baine, the principled leaders of the Horde. The Horde needs its antiheroine. But Sylvanas' pragmatism (which she has always demonstrated, with everything from Varimathras to allying with the Horde itself) is in danger of being subsumed by a personality that she doesn't even have, one that ignores practicality in favour of brutality and sadism. Taking down Koltira? Appropriate. Allying with the Val'kyr? Logical. Plaguing Gilneas? Harsh, but effective. But lets not go any further. She needs to get back in touch with her song, her family, and while avoiding a complete heel face turn, get off the path to complete character 2D. I'd suggest offering the Gilneans back their city (since apparently we're not doing anything with it), but she's rabidly defensive against potential threats, which makes sense. Maybe another concession, suggested by her sisters. Something that indicates character development.