Yes and no. Yes, there will be other Hero Classes in the future, but the Old World classes form the basis of the class distribution. And no, even though the hero classes are supposed to be stronger and special compared to the original classes, they cannot be because of the delicate issue of game balance! And this -compared to the whining which started already before tBC that the levelling takes too long and is boring and bla-bla-bla- is guaranteed to bring the pop-up levelling to the original classes. Whether it is a new starting zone or the opportunity to start from a higher level, it doesn't matter, it will come. That's my God's honest opinion.
Granted, Blizzard has already paved the way for this by lowering the 1-60 levelling time considerably. This, in change, results that you cannot even imagine completing all the areas in proper level range, as you outlevel the quests constantly! It's really frustrating to notice all of a sudden that you have only grey matter in your quest log after one successfull instance run (ding twice, yippii!) and that you have to do some to be able to continue the questline. Hacking through more grey matter only to proceed isn't too gratifying, let me tell you.
This effectively lead to the Blizzard's Secret MMO in the making.
I don't see them doing something controversial or new in the sense that we haven't seen it before. After all, Blizzard is more known to take others brilliant ideas and honing them to the polish known to be Blizzard. Warcraft, Starcraft, Diablo, WoW; all are great examples of that. And don't even mention the orcs... There is a reason why Games Workshops orcs are green, and even it has been copied without a thought...
What I see is WoW2. Why? Like Danshir posted in his brilliant blog, lorewise the Warcraft world is soon in a void of archenemies. The lore-savvy people are already second quessing the who-what-how of the next big instance enemies after the Lich King: who is there to pose any threat to the world or planning the world domination after the high leaders of the Scourge are gone?
This is where WoW2 comes in. Set in either future of the WoW or to the turmoil before the Warcraft series, it gives the answer to the question: new era, new enemies, familiar world and game functions, only refined to the current standards.
It's common knowledge that the engine behind WoW is outdated: the low end computers today are of higher specs than the high end ones when WoW came out. It needs fixing, especially when you look at the slowing down of the game due to patches upon patches.
The counter argument was that this new WoW would kill the continuity of the characters, and kill the roleplaying aspect of the game. Nonsense: it would be a different game, different reality, different characters. Sony did this with Everquest, and had there been no WoW, EQ2 would be THE game. At least I believe so, and I'm not the only one (/wink Tipa).
Sure, it could be Starcraft, but thinking of the difficulty of making a successful sci-fi genre MMO I doubt that Blizzard-Activision is taking this chance. Fantasy >> scifi in popularity. Sadly so.
Like I stated earlier, Blizzard is genious in copying and tweaking the ideas of their competitors. SOE has shown that it's possible to make a 'sequel' to a MMO, and Blizzard has had well enough time to device a plan to get the best of this. There wouldn't even be a need for the next expansion, if planned correctly: the game would be the expansion!
Enough. I'll go on pushing my pixels in the Old World. There's enough content for me for YEARS!