Star Trek had a spin off to another show that wasnt star trek
I know its not a sitcom, but the original Star Trek back in the 60's had a show that was intended as a spin off to a totally new show that took place in modern (then) times.
The pilot was the episode Assignment:Earth and it included a guy from the 60s named Gary Seven who was taken by aliens in the 60s and made a super human and put back on earth to save earth from its dumb decisions. It even had a very flaky Terri Garr as his assistant on earth.
It is so obvious it is a spin off, and the way they worked it into the ST universe was totally dorky, and just didnt work.
Well the show didnt either.