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.
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.
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.
On some days during the shoot of the first season, Fox would often hitchhike from Brentwood to Hollywood, because he had no car.