These were talked about last map as being a third tier class that you would have to master multiple classes to spec as such. I believe the the concept was scratched due to balance concerns and not being worth the coding effort.
I dunno how they would have been done in the previous maps, but, to be honest, there aren't any balance concerns if you do it right. Just because it requires you to master 4 classes before "unlocking" it doesn't necessarily mean it has to be stronger than the others.
It's very simple to just create 4 new classes, balance them, and then stick the "Legendary" title on them.
I personally really like the idea of legendaries, and would love to see them make an actual appearance on HC. I've offered to create new classes before, but I think Kainzo would just prefer to focus our manpower on other things right now.