I would really like to adapt my favourite book, Michael Chabon's Amazing Adventures of Kavalier and Clay. There have been lots of plans to do it (see here: http://en.wikipedia.org/wiki/The_Amazing_Adventures_of_Kavalier_%26_Clay#Film_adaptation) but they've all failed so far. I'd cast Adam Brody as Kavalier and Michael Cera as Klay, but swap them for other actors as they get older. I would definitely keep in the World's Fair scene which they've mentioned cutting out. I'd do the comic scenes as animation, in a fantasy world put together by Jamie McKelvie and Kieron Gillen.
My other plan is to create the next British blockbuster kids' series to replace Harry Potter, by doing a squillion dollar adaptation of Brian Jacques' Redwall (looks like there's been a tv cartoon adaptation before but not much else: http://redwall.wikia.com/wiki/Redwall_Film_Adaptations) I'd do it as real backgrounds with motion captured actors, then CGI them all into mice/weasels/etc. I don't mind if they become American - perhaps the baddies can be British in classic style? Maybe Tobey Maguire as Matthias, Andy Serkis as Cluny.
HOLY SHIT THERE ARE NOW 22 REDWALL BOOKS! I think I only read about 6 when I was a kid.