I think that casting legend would be a very difficult task, but if this book was made into a movie, i dont think i would select any of the holywood A class actors. For a story like this it is more important to select actors that are closer to the actual characters than those who have big names as movie stars.
Want proof? consider Troy and Alexander. These stories are spectacular, but have been destroyed by holywood making movies based on hiring huge stars like brad pitt, and colin farell, and big cgi effects. They dont put enough money, time, effort, and thought into the script, and i would hate to see any of david gemmells novels to go down the same path. i would rather have no movie, than a bad movie.
And personally i can not think of any actor i have ever seen that would be able to play druss convincingly.
If it were my choice in casting Legend, i would be scouring the globe looking for convincing, no-name actors, who can actually act well!!