Michael J. Fox's first audition was deemed terrible by Gary Goldberg because Fox came off too smart-aleck. Casting director Judith Wiener liked Fox and begged Goldberg to see him again. Fox took a different approach; the audition went great, and Fox was offered the part.
The cast wanted to end the show with the entire Keaton family dying in a plane crash. They didn't want to do a reunion show in the future.
According to Michael J. Fox's last Spin City (1996) episode, Alex P. Keaton became a Republican Senator from Ohio.
Mallory's boyfriend Nick was primarily a clone of Fonzie from Happy Days---complete with his trademark "Hey!" and leather jacket---and only intended to be a "one shot" character. Scott Valentine made a strong impression on the show's audience and producers, making Nick a recurring character. Nick proved so popular that the producers tried to give him a spin-off series, though the pilot never sold.
When Meredith Baxter became pregnant, during the hiatus between the second and third seasons, her pregnancy was written into the show. However, she still was written out of several episodes after giving birth to twins in real life.