The film is a prequel to the 1939 The Wizard of Oz, and the offer has gone out to Downey, The Playlist reported.
The family friendly project (duh), previously known as "Oz," is set up over at the WB's New Line banner and is based around a script by Darren Lemke ("Shrek Forever After," "Jack The Giant Killer"). Naturally, details on this are tight for now.
The movie is one of two proposed Oz-themed movies in the works.
It's unclear on the timing of this movie, as Downey's a busy boy. At a minimum, he's committed to a second Sherlock Holmes movie this fall and Joss Whedon's The Avengers early next year.
For his part, Mendes was slated to do a 23rd James Bond movie, but that looks to be on hold as a result of MGM's financial difficulties.
Downey's Iron Man 2 opens May 7.