It would be great to see some old heroes make comebacks.
I think Henry summed it up well. He mentioned about 15 heroes. We have around 85 heroes in the game. So 70 aren't even worth a mention.
Passives, low additional damage values, poor synergy etc are reasons why so many aren't relevant today.
Perhaps changing passives skills to percentage values would be better?
I like the idea of giving the skill increases a boost in higher levels. Perhaps a legendary status could also give a boost to other skills.
Other heroes could get legendary status for the stats/skills boost, and skills could even be added later.