The tortured history of the TMS feature film Little Nemo: Adventures In Slumberland (1992) could rival that of Richard Williams The Thief And The Cobbler. It was an American/Japanese joint project, with no less than Hayao Miyazaki and Isao Takahata involved in the pre-production stage (1982-83).
George Lucas, Chuck Jones, Gary Kurtz, Ray Bradbury, Chris Columbus, Moebius, John Canemaker, Leo Salkin, Paul Julian, Ken Anderson, Frank Thomas and Brad Bird (who talks about his involvement in the comments below) were attached to this film at one time or another. Bill Hurtz (George of the Jungle, Unicorn In The Garden) and Masami Hata (Sea Prince and the Fire Child) ultimately directed the final release, admittedly a mixed bag.
The idea of making a fully animated adaptation of Winsor McCay’s comic strip masterpiece somehow seems like a good idea (McCay himself authorized a musical stage play based on the strip in 1908), and the names assembled (above) to tackle such a project were certainly capable doing so.