Babylon 5, the way JMS originally plotted it out: Keep Sinclair, keep Lyta, don't end it at the end of season 4, etc.
I've also heard that the mole was supposed to be Takashima, not the blonde telepath (blanking on her name).
Basically keep the network suits out of the production process. If an actor had to bail, replace the actor but not the character -- just don't mention the change.
"For shame, gentlemen, pack your evidence a little better against another time."
-- John Dryden, "The Vindication of The Duke of Guise" 1684