It would be great if the Legacy Dragon would appear on the breeding island. I've been trying for a while.

It sometimes does. And sometimes you can buy it for gems.
But if you keep breeding pure x pure or a legend x legend, you will eventually get legacy and all the other legend dragons. I used pure x pure because it was faster.
It has taken some players 2 years to breed legacy. I got it in my first week of trying. Just keep at it.