Wether it's to introduce new people to it, missed opportunities or just flat out because it has to.
My picks would be the I am number four movie based on a book of the same name just because I feel it would be the next big book series if done right and makes it past the first film. Also the Percy Jackson series in a more book accurate way and lastly avatar the last Airbender the movie was unwatchable for me but I feel a reboot with better actors and better effects (which are entirely possible look at any supernatural movie released) would blow up if done right especially with the already existing major fan base.