9. Stargate SG1
Stargate SG1 was cancelled in 2007 after ten seasons and fans felt the blow. Even though spin-off show Stargate Atlantis was still going strong, the loss of an 11th season for SG1 was saddening for many. There of course had been plans for an 11th season, which would have followed the same rough trajectory as the 10th.
The final season had focused mainly on the quest for the Sangraal (a small red orb that could wipe out Ascended Beings) and its eventual use on the Ascended Ori, leaving on their mass of human followers ravaging the Milky Way Galaxy. The 11th season would also have focused on the search for another mystical artefact left behind by the Ancients, the mythical Ark Of Truth, which has the power to end the conflict.
Fortunately for fans of the show, two direct-to-DVD movies picked up where the show left off. First up was the titular Ark Of Truth, detailing the end of the Ori war. Then came Continuum, where Ba'al attempted to have his last laugh. Throw in a few more light-hearted episodes with a couple of jokes and hopefully a cameo by Jack O'Neill, that probably would have been the 11th season.