I understand the idea of the post. It is quite entertaining to watch everyone freak out about wow. I would love to see PVP as good as wow had back in BC. I would love to see a player driven economy as incredible as Eve's. These are not the things the OP is asking about though. He is asking about artwork, encounter design, lore possibilities, and other non gameplay affecting ideas.

Story wise I could see the Burning Legion attempting to recruit the demons that inhabit our world. I would love to see Felhounds and Doomgaurds tearing things up in Gridania and not to mention the Infernals that could rain down. There could also be some cool possibilities with the elemental lords. Ragnaros teaming up with Ifrit, Titan and Therazane, Al'Akir with Garuda. Those could be some really cool encounters. The Old Gods(which are not specific to wow) and Algalon the Observer could also make things interesting.

Some similarities that could be exploited are how close some classes are to each other. Obvious ones are warrior, paladin, ninja/rogue. Some of the others are Conjurer/Shaman, Summoner/Warlock, Black Mage/Mage, and White Mage/Priest. The druid class share some similarities to Conjurers, and Astros but that is a bit of a stretch. Mechanically the Hunter class shares things with both Archer and Mechanist.

Some gear I would like to see are the more unique weapons from wow. Thunderfury and Ashbringer would be awesome to have. Some of the lesser known ones that I think look really cool are Bulwark of Azzinoth, Apostle of Argus(and its PVP variant), Quel'Serrar, Zin'rokh Destroyer of Worlds(what DK wouldn't want that sword?), Timeslicer, and Terestian's Stranglestaff.

I assume many wow veterans would love to see a variant of their old tier 1-3 armor sets. The Paladin's Judgement set, the Warlock's Plagueheart set, and the Warrior's Wrath set are all pretty amazing looking.

Anyone could come up with a story for including these type of creatures and events. That's what these companies do, they make interactive stories. Would you really moan and complain if they made a good and enjoyable story just because it was World of Warcraft?