I wish it was clearly communicated if a series was fully planned and with a final ending, or not. The best series I know are selfcontained, single season series with a clear overall story telling.
J. Michael Straczynski had a a storyline designed to last 5 seasons (and apparently with several "trap-doors" for when actors left the show etc.), but the studio then let him know that they were going to cancel it at the end of season 4, so he had to tie up the main plots quickly. However, the studio then decided to produce a fifth season, so Straczynski had to string together a whole season that he hadn't planned for.