I'll present mine as a more idealistic and optimistic outlook for Arthur and his friends...
Here we go!
Arthur: Probably a librarian or book publisher - he likes his books.
D.W. : Producer of television networking and/or advertising.
Buster: Fast food worker, albeit not necessarily restricted to the one who fries food all day.
Muffy: Inherits Crosswire Motors, and expands it to include a fashion department.
Brain: Top ranking scientist that cures the common cold.
Francine: Superstar basketball player.
Binky: Part time ballet instructor.
George: Probably industrial manufacturing, maybe woodwork.
Mr. Ratburn (although he's already a teacher): May rise up to principal amid Haney's inevitable decline.
Fern: Writes poetry and perhaps novels as a living.