I think an adaption of the original Johnny Quest cartoon could be as big as Harry Potter.
The Herculoids is another sci-fi/fantasy toon that would be great as a live action movie...
There's a bunch of fantasy novels that are just BEGGING to be brought to the big screen;
1) Edgar Rice Burrough's John Carter Of Mars. This has been tossed around Hollywood for two decades now. Always seems to get shelved at the last minute, unfortunately.
2) Roger Zelazny's Chronicles of Amber. This is like The Bourne Identity meets Lord Of The Rings. Could be epic.
3) Piers Anthony's Xanth Trilogy. Anyone ever read these? A cool fantasy novel so far removed from the traditional LOTR -type story, that audiences might really love it.
Also; i know it's in talks, but a GOOD Conan movie would be nice. Hollywood needs to actually read the books first, and realize that Conan is more than just a grunting, muscle bound idiot. The trick is finding a body builder who can actually act. A faithful adaption would be wonderful.
Just my two cents.