What are Heroes?
Classic Warcraft 3 characters. Either didn't fill enough of a primary role in the original WoW or didn't feel right being played from level 1.
Epic feel -- should be very different from the other classes, such as the new resource system for the Death Knight.
Lots of ideas
Forking classes from current classes or additional talent trees. But no, because the character classes wouldn't feel distinct. Would all be watered down.
Multi-classing -- a secondary class or a hero profession. For example, a warrior would become a deathknight. Would water down the different character classes. How is a warrior death knight different from a warrior death knight, etc.
What is a mage deathknight or druid deathknight -- wouldn't feel right.
Class morphing -- the character class would stop being the old class and then do the quest and become a deathknight.
Players would feel they were losing existing characters. What if you don't like the new one. So they decided to unlock instead of create new.
Because tanks are hard to find for groups, they chose Deathknight as a potential to alleviate this.
Class balance concerns
Balance implications
Many new heroes could be too complicated