The big question for an animated series is who owns the toy rights? Lack of toy sales is what killed Young Justice on the DC side (largely driven by 2 factors from what I have read - the toys quality sucked, and were marketed at boys even though the show had huge popularity with girls - the marketers weren't willing to adapt) and if Fox gets the money from any X-Men toys then there is no incentive for Marvel to make an X-Men animated series even if they retain the rights to do so.