Too many balancing issues. Not impossible but probably not going to happen until they either A) work everything out with current balancing issues, or B) need to throw people a Demon Hunter to save subs.

Well, following the pattern of expansion, we've had:

BC- new races,
WotLK- new class,
Cata- new races,
so MoP- new class.

Just following the pattern, it seems likely. But then, Blizzard does love to mix things up. We might get an epic new profession instead. Or something.

I never got why DKs were considered a "hero" class. Other than starting at level 55 or so, they don't feel any more heroic than any other class.

It's just a left-over term from when Blizzard planned Classes to be upgradable at max level.

If we do ever get another class, I hope they start it at level one. It would be nice to give you an option to start out any class at 55. They would have to make another starter zone. Perhaps something more tutorial to your spec rather than class. From the whole balancing issue, I can see why they wouldn't want to go that route with a new class.

Among all the other questions like "When will the next expansion be out?", I wish the definition of a Hero class would be put down in a FAQ, and linked every time someone asks.

The exact definition of a "Hero class" by Blizzard is a class which starts at a higher level, has a customized starting zone, and gets a free mount. Nothing more.

The term originates from Heroes, from WarCraft 3. Powerful, unique units each race had. They were the primary unit your race would use, and could level. If killed, they could be resurrected. Many of the abilities we have in-game come from hero units. Fan of Knives? Warden, of which Maiev is one. Metamorphasis? Demon Hunter, of which Illidan was one.

Almost all faction leaders were hero units in WarCraft 3. Jaina Proudmoore, Sylvanas, Thrall, even Kael'Thas.