If you don't think this would be one of the greatest books even written in the history of literature, then my friend, we have nothing more to say to each other.
There's a whole website full of these for those interested at docfantasycovers.com/ No My Little Pony:Friendship is Magic covers as yet, but one can always hope.
But which villain would Doc be facing? Discord? Sombra? Chrysalis?
Or should we just go right to the top with:
ENDLESS DARKNESS! What is the source of the unending night that has fallen across planet Earth? Some call it Doomsday; a cult devoted to the mysterious Nightmare call it the birth of a new age of glory and terror! Doc Savage alone knows the truth, that the Night Eternal is nothing natural, but caused by some unearthly power from realms beyond humanity! With the aid of a strange young woman named Twilight, Doc and his friends must strive to save a terror-stricken humanity from both itself and an eldritch menace from worlds more alien than even Doc Savage has ever explored before -- the menace called NIGHTMARE MOON.