This week's Blog Azeroth Shared Topic is New Class Theories, suggested by Shadowfox. See other responses to this topic here.
There have been many theories on what the next hero class will be, but at this time all signs point to Demon Hunter (which is consequentially what Shadowfox wrote about). Why else would the Writings of the Void quest chain exist?
However, if I had to choose what the next class would be I'd definitely pick a Monk type class. I discussed this briefly in my review of EQ2, found here. Typhoon Andrew also has written about it here.
When I first started gaming I played Baldur's Gate and eventually Neverwinter Nights. Just like in WoW I tried a little bit of everything, but in the end settled on the Monk class as my favorite. It was a disappointment when I started playing WoW to find no Monk class in the game. There is something just amazing about beating someone down with just your bare fists or a simple stave and the sheer power of your will, dancing out of harms way on fleet feet. Of course having the ability to heal yourself and others helps too. Everyone knows I'm a healer at heart. Monks are like the perfect combination of rogue/priest.
On the other side of the spectrum are those that long for a Battle Mage of some sort. A plate wearing caster, that can get into the thick of things without dying, but still cast powerful spells at range. Some have suggested a buff class, such as the Bard in EQ. There are so many ideas and options, none of them all that unique or new to the fantasy rpg genre.
In the end, as I said at the beginning, all signs are pointing to Demon Hunter. Time will tell...