Ugh, to make an acceptable list of legends would be exhausting af, just the omissions I can think of
Billie Holiday as rock... Ok, but you could add the jazz section what would allow you to mention many other legends (Miles Davis, Charles Mingus, Louis Armstrong, Herbie Hancock, Nina Simone, Peggy Lee, Ella Fitzgerald etc) and even more if you combined it with blues, soul and folk. But since ATRL doesnt care about this kind of music, its understandable that you didnt.
Anyway, I wouldnt call them legends, but from those mentioned in OP, Celine, Barbra and Mariah. They can sing, but the music they put out is either meh or just awful.
The only country legends that actually mean something outside US are Johnny Cash and Dolly Parton.