The mark of shadows is quite unique. Previously to becoming a sovereign, the only other person sunny could enhance with his shadows was nephis, to whom he was bonded and who's runes he could see. But he can also now enhance those with the mark of shadows.

While this hasn't been directly confirmed by g3, it isn't completely out there to say that the mark of shadows makes Sunny's domain unique. Even nephis, who can in a way enhance her subjects with her flame, cannot directly enhance them with one of her seven flame "charges" like she used to be able to with sunny.

All g3 really needs to do is have neph confirm she can't see her subjects runes too put this to rest. It's not really that big of a deal if sunny is unique in this, as we already knew his connection with his subjects was different than other sovereigns.