I think any spin-offs should be left entirely to fresh blood, and all of them ought to be some ways down the road. Just look at the quality of the second Star Trek series, which began two decades after the first. I don't think that all of Springfield's best should get their own shows, but if it was going to happen, here is my take:
"Simpson and Son" Old Homer and a mature Bart have gone nowhere in life, and are now a Yellow-American father-son team living in a derelict slum, where they operate a wrecking ball company. Typical zany fare, Lucy-esque for the most part.
"Brenda" Marge supplies the voice of a sassy Jewish chick named Brenda Morgenstern who lives in New York City. With the additional talents of Krusty the Clown and the Rapping Rabbis. Situational and character humor.
"Daughter of the Sun" Lisa, after a brief stint at college, goes about taking over the world one week at a time while making herself seem like a sweet, compassionate young woman who cares about sea junk and trees. A drama, with moments of wit.
"The Simpsons: The Next Generation" Maggie has turned out a lot like Homer, and has a family of her own in the anytown land of Bellingford. Who would have thought?
"The Hibbert Show" Julius Hibbert is a well-to-do doctor living in the New York area. His large, jolly family and his own unique brand of humor provide a mechanism for each episode's thinly disguised moral lesson.
"The Bus that Couldn't Slow Down" Hans Moleman is making action films again.
"Al Dente Youra Head, You Kookalamanza!" Given a show on the TV Food Network, Luigi spends every episode insulting his audience.
"The Fantabulous Misadventures of Sea Captain Horatio McAllister" A Nautical Network exclusive, this adventure/comedy features the good captain as he sails the world in search of ever more fantabulous misadventures.
"Keeping Abreast of Internal Revenue Policies" An older Martin Prince hosts this informative program that details with inordinate meticulousness all the updates and revisions to the forty-four thousand page U.S. tax code.
"Guest Starring!" This show provides a funnel for those inevitable calls from the fans for famous guest stars, as "Guest Starring!" features guest stars, only guest stars, and nothing but.
"102 Dalmatians 2: 103 Dalmatians" Santa's Little Helper gets a paint job for this demanding film.
"Ned in the Holy Lands" Ned Flanders demises, but before he can get through the pearly gates, he must relive one Bible story each week, all set in the psychotropic stylings of "Alice in Wonderland," and each time must confront yet another Biblical self-contradiction. This show focuses its appeal on the stoned youth demographic.
And so forth.
~Josh