JMS planned out 5 years/seasons of B5* but he lost his notes for 5 and it became a jumble due to that and a quick uncancelled.I wish shows could write for the story, not to fit some pre-set length. I have seen shows that should have been 6 episodes stretched out to 18 or more. And I have seen 10-episode seasons that needed more room to breathe and expand. I tell you, once I get that ring of wishes...
Along with having in-story reasons in case actors had to leave for whatever reason.