I´m tired of people bringing up the reboot/restart idea all the time... Put Batman Begins outta your minds, guys. That was a special situation. They rebooted cuz the franchise went south both critically and commercially and it was totally inconsistent in tone and feel. The X-Men franchise, for the most part, did work critically and all three movies were box office hits. From the studio´s POV, there´s no reason to get back to square one. Certainly not a niche group like the hardcore fans of Cyclops or Storm. Like I said in other occasions, if they make X4 they can surely find a plot device to say that Cyclops didn´t die in X3 and bring him back, and there were seeds in the end to bring Magneto´s, Mystique´s and Rogue´s powers back, so these characters could very well still be used.
A more productive thought is what good storylines haven´t been used yet? Days of a future past, anyone?