A hero(es) that is not any of the five existing colors, but is a 6th element called “Dark Matter”.
However, this would not add another color to the board because their tile attacks and mana generation would come from otherwise regular looking tiles. You would not be able to see the Dark Matter tiles, they look like any other tile, you would only know if you’ve fired a Dark Matter tile when you get the mana.
This could be a way to enter into 6* hero territory. Or just another way to diversify existing tiers. They would be a huge gamble since all their tiles are unknown to the player, but ideally once you’ve filled their mana, their attacks could be massive.
Anyway, just bored at work.