Feedburner shows a surprising number of hits to my blog today coming from people searching for WoW bards. It turns out that this is the "official" Blizzard April Fool's Day joke, an announcement of a second hero class: the bard. Who, due to the Activision Blizzard merger is going to be played using the Guitar Hero controller. :)
Sadly that is selling the idea short. Just yesterday I was talking with a player who has a druid, and who finds the idea that the next hero class could be an archdruid not fun at all. If a hero class is sold as being a more powerful version of an existing class, the existing players of that class get upset. It's a much better idea to create hero classes that are totally different from existing classes, and a bard would have fitted that model well.
But however you call or market them, new classes will almost always be perceived as threat from existing players. Not only due to fears of the new class being better than the old one, but also because of competition for the same loot. I have no idea how a deathknight compares to lets say a dps warrior in power, Blizzard is probably not even finished designing them yet. But I do know that the deathknight will start at high level, around 60, which will reduce the interest in creating new warriors. And I'm pretty certain that a deathknight will roll for plate armor loot with strength and stamina bonus like a dps warrior, and maybe even for plate armor with stamina and defense bonus like a tank warrior. You can easily extrapolate those problems to archdruids, archmages, or whatever else hero classes you can think off that are based on existing classes. A bard wouldn't bother any existing class.