They had a fantastic opportunity to do that in tears, and honestly still do if nintendo decides to to a big dlc. It would just be a lot of work.
Since Zelda is stuck in the past, we could totally get a glimpse of ancient hyrule by getting to explore as her. Just port the topside map again with changes. People would love it.
The biggest problem would be giving her a unique moveset. I think it would be a huge injustice to make her just an alternate skin of link, to both of them.