It's official. NBC has cancelled "Heroes."The network seriously mulled bringing back "Heroes" for a shortened final season to wrap up the serialized show.In the end, given the fairly high cost of the drama, the show's consistently declining ratings and the number of new hour-longs coming to NBC next season, the network decided the UMS-produced series wasn't worth an additional season. Sources say the network is still leaving the door open to conclude the show with a special or movie.The show has long had a rocky relationship with its fans. Out of the gate, "Heroes" was a big hit for the network -- an ethnically diverse big-concept ensemble that performed well overseas. It was NBC's "Lost."But the show's writers struggled to keep the show's twisting narrative on track. Characters were killed off and resurrected with regularity. Narrative threads were started, then abandoned. Unlike "Lost," there didn't seem to be series-long central questions driving the show that needed to be resolved with a final season.