To begin, I really do not have anything against Hisui. She is a side character with very little screen time. However, I feel like her character could be so much more. My fixes would probably mean more screentime for her or less of a focus on the story parts she is in.
Considering we know so little about Hisui, I don't mind her design. I prefer her with her hair down but I enjoy her later outfits more. Though I do wish she had more of a ball gown style approach to her outfit. My first change would be more character change. The approach I see her in would have her in a white ball gown maybe with gold and white with some mint green and red. My other idea could suit her current outfits.
I do love the princess as a character with her personality. Though, with my second approach to her character she would be a little less lady like, or go exploring. Ideas like she actually does like training with magic and isn't afraid to stand for her country.
Honestly, my biggest gripe against poor Hisui is... Her magic. She is one of three celestial mages left and we are barely reminded of it. She has had her moments, but not many. I honestly see her possibly being like a foil to Lucy. Lucy left her life of luxury and strictness to live the life of wizardry. This was freedom in her eyes after losing her mother and her father became greedy. Meanwhile, Hisui could've had a chance to leave an seek a wizard life after learning Celestial Spirit magic from her mother (We can say her mother had died during the genocide of Celestial Spirit Wizards). She decided to keep her life of responsibility and choose the safety of her people over her personal happiness. She also continues to strengthen her relationship with her father.
As I said, I do not hate Hisui. I just found her to have potential for a good backstory but, unfortunately, we didn't see much from her.