I think at the end of the day is all subjective... she's gonna come up with covers and people are gonna like them and hate them too... so I don't believe in an ideal single cover.
I don't expect her to do anything I've got in mind because that's all in my mind anyway and she can't read it xD That's why I would love to become a photographer and take beautiful ayu-photos
In terms of photographs, I like and enjoy simplistic pictures as well as fancy ones with lots of colours... I think I enjoy and like most of her single-covers, if not all of them O.O