I wish we could have gotten the 'real' final season of Babylon 5 rather than having most of the show's plot resolved already in S4 due to the treat of cancellation. And also more than half a season of Crusade. And a Legend of the Rangers series. And the telepath war movie. And the Memory of War movie. And more than one Lost Tales movie...

(Babylon 5 fans: getting our hope crushed over and over again since 1997)

Also, I would have loved loved loved to have had a second season of Twelve with Bill, Nardole and Missy because they were an absolute delight together (don't get me wrong, I loved Clara & Twelve too, but Bill deserved a second season)