The reason Death Knights were a hero class, is because it felt right. It was supposed to feel liek you were already a hero, and becoming a Death Knight was the next chapter in your story. Well, what about the next chapter of a Druid's story, or a Mage's?
What if, instead of a new class, they added some sweet changes to classes at level 55, turning them into their Hero Class?
A revamp of old Outland.
An epic quest, from levels 55-58, leading into the new Outland.
One for each eace/class combination, towards the same goal for each race, like with the starter zones.
You can start any character at level 55 as one of these "Hero" classes. They start in the zone for the epic questline, and can't leave it.
It's basically a new starter zone for each of the 13 races, with a lot more large-scale things. It's expected that, in the human questline, you're a High Priest, or Archmage, or Farseer.
Let's say, for the purposes of the argument, that the Burning Legion has returned to Outland, and is swarming through the Dark Portal, and is all over Azeroth now. There would be two parts of each questline. One in the capital city, race-specific, and one at the Dark Portal in Azeroth, faction-specific.
The quest text and givers would be different depending on your class, but the outcome would be the same. When the Burning Legion returns, it returns big. And badass.
You're a human High Priest. You're in Stormwind. The skies turn green and black, and infernals rain from the sky. You have to get as many civilians into the Deeprun Tram as possible, sending them to the supposed safety of the Ironforge Dwarves. Aid the other heroes by healing them, killing off the demons, or tanking them. Then hop on a mount and start riding towards the blasted lands, fighting massive threats on the way. Maybe join up with some Worgen on the way. When you get there, all hell is literally breaking loose. Demons are pouring through the dark portal in uncountable numbers, to be met with Dwarven tanks, Gnomish airships, Goblin shredders, Orcish catapults, Elven glaive-launchers, countless heroes, from all races of Azeroth, fighting against a seemingly endless horde of demons. Turalyon and Alleria are there. Yes. You join the fight, and it has some epic cinematic end, with tension building from Horde and Alliance.
Then receive your medal from some notable character. You helped in some grand quest, just like the Death Knights did.
What's awesome about this is that you'd see all factions present here. Not just Horde and Alliance, but Kirin Tor, Sunreavers, Blood Knights, Sunwalkers, Silver Hand, Orcish Warlocks, Gnomish Tinkers?
The point here, being that everyone has something to protect.
And you aren't some noob starting his adventure. You're powerful. And respected. And willing to aid something as great as this.
You're a Human Archmage.
Or an Orcish Farseer.
Or a Tauren Sunwalker.
Or a true Blood Knight of Quel'Thalas.
You would have a choice, then, when you create a character.
Is this character starting his adventure? Is he going to watch as his city falls to the Worgen curse and Forsaken assault? Will she start her adventure by helping cleanse Teldrassil? Will she be called to action when Troggs are suddenly displaced from their caves?
Or will your character have seen all of that already?
Maybe your character is a war veteran, or an old wiseman.
Maybe your character is ready for something a little more...