I would definitely like there to be more hero cards, because they really make the game more interesting.
However, I recall reading somewhere that they said they'd prefer to only have 1 or 2 hero cards at a time, to emphasize them for their respective expansion. I don't have a source, but I think it was something they said.
The last time they did a hero card for each class was back in Frozen Throne, since then it's been staggered. So I wonder if they'd do it again this time.
That would definitely be a cool way to round off the journey of the mercenaries. I'm down for it.