Merrick here...
THE HOLLYWOOD REPORTER says the Robert Downey Jr. has "fallen out" of OZ, THE GREAT AND POWERFUL - the Sam Raimi directed pseudo-prequel chronicling how the man who would become "The Wizard" got to Oz, and (presumably) became the Wizard.
Disney/Raimi/Etc. are now said to be in negotiations with Johnny Depp to assume the role. Which seems like totally fine casting, although perhaps a bit too uninspired and obvious.
Frank Morgan portrayed the equivalent role in 1939's THE WIZARD OF OZ - actually appearing as several characters in that picture.
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