well Wonder Woman and Teen Titans would be my top choices.(please check my post in the teen titans thread for my story/casting ideas)
But i have another idea as well...and i'm sure i'll get plenty of laughs for suggesting it....but i think the original 16 part Amethyst maxi-series would make a pretty awesome movie.
with the FX we now have Gemworld is totally doable and IMO could look amazing.
i also think it would appeal to many different demographics...fantasy fans, magic/sorcery fans, superhero fans, tween/teens.
call me crazy but i think if done properly it could be a huge success.