Of all the serious spin offs, this is probably the most extreme (Unless you somehow think "Lost" is based on "Gilligans Island", I do). With the popularity of their cheesy Christmas reunion movie, the Bradys were greenlit for a "Dramedy" series on CBS in 1990.
Nobody got a football in the nose this time, it was straight up drama laced with a little bit of that Brady humour. The show got announced on Oprah (!) with all the cast members present. Marcia dropped out this time and was replaced by another actress.
I seem to recall a storyline of Bobby getting in a racing accident and suffering possible paralysis. Nobody wanted to watch that and the show went away after six episodes.
Theories on it's cancellation mention Robert Reed's illness but I personally just think the whole idea smelled.