A hero based on Arya Stark’s faceless ability … the special goes off and the hero takes on the form/power/special/mana level of the enemy’s hero with the most mana built up. They stay in that form for 3 turns.
A hero that is based on being a thief/bandit. Their special goes off and they steal 50% of the targets existing mana or health… which ever is lowest. Maybe they also steal 25% of the existing mana or health of the nearbys.
Similalry, how about a Robin Hood character that steals mana from one enemy and gives it to the weakest ally?
A hero that has a twin. If there is an ally dead when their special goes off, they have 35% chance of summoning their twin in the empty space.
A hero that is based on a chameleon. When their special goes off ,they attack and change color. Or, maybe they just change color every 3 turns.
A hero that is “mixed race” of yellow/red. It could be a super rare orange hero that cuts in half the damage from purple/blue and doubles the damage to those colors.
How about a hero with a disability? Disabled people can be heroes too 
A hero that is surreally skinny and that can only be hit on one (or maybe two) widths of tiles instead of the normal 3.
How about a shape shifter? Their special goes off and they turn into something else for a few turns … i.e. werewolf, the hulk.
How about a super, super rare hero that is all 5 colors… sort of like the Voltron.
That’s all for now, but I am very interested in a job on the creative team