When everything has already been done, from inter-planetary travel to exploring ancient cities - both real and mythical, from martians to mammoths, kings with possessed rings, a boy wonder with a wand, and so many other things.
What do author's do that really makes their story one of a kind, when every story imaginable has already been told?
A great author writes their story connecting the dots in a unique way, adding their own original flavor to everything they say.