I think the game would be alot better if they returned to the vanilla/tbc style of inaccessible content. So that lesser players can have something to aspire to, and so that world first ans what guild in the world is #1 and stuff like that starts being interesting again. That adds an aspect of quality to the game for both casuals, who won't experience the content, and the hardcore who will. But - unlike the vanilla/tbc style - they should continue what they are doing now and keep focusing on the casual players and release tons of stuff for them to do. Basicly they should do both - and no, they are not doing both now. Heroic raids and challenge dungeons etc. are not the same as releasing hard core content. That is just buffing existing content making it harder, turning the game into a grindfest rather than an epic adventure.
What they need to do is have raid content that is not the same with a bunch of difficulties, but rather they need make raids with 1 difficulty that are truly challenging and hard the way they used to be - and then they should also have casual content. Start making like 10-man casual raids and 25-man hardcore raids, and if the endboss of the game is Azshara well then she will obiously be the last boss in the last 25-man raid, and then have a few 10-man casual raids where some Lady Vashj-style lieutenants of her are the last boss - that you have to defeat because they are guarding the Queen's Citadel or something.
Basicly, they need to do less difficulties and more content. And vary the difficuly on the content. Introduce legendary items as the item drops from 25-man raids, keep epic items for 10-man raids and keep rare items for 5-man dungeons. And make more 5-man dungeons and less daily quest hubs, too.
Some kind of combat system for word PvP would be cool too. Like, if a bunch of people clash out in the Barrens some kind of semi-Arena game with automatically launch and x Horde and x Alliance players in proximity to eachother / in party will automaticly be invulnerable to outsiders as they fight to the death - and the team that wins will automaticly have all their fallen instantly revived as they win and their HP go up to 100% (as to prevent spectator killing off the winner should he have like 10% hp left as the game ends). Stuff like that.
Also, more attunment. They say they didn't like attunments because it was.. what?... I don't remember, something about complicated or time consuming or something? Yet since then they have introduced daily quests which is perhaps the most boring piece of crap the game has ever seen? (although I like it being part of the game, for casual players mainly, although - as all things - it should be kept to a certain level of moderation) ... Basicly make it harder to just quit the game, come back after 7 months and jump back in where everyone else are. Just a tiny whiny bit harder.