You may think this is weird, but I think that Arthur will have a flash forward where D.W. and Brain get married, no spin-off necessary.
Even though in "Kiss and Tell," D.W. wanted James to kiss her, and in "D.W. Unties the Knot" she told Muffy that the groom was "King James," there have been moments that led me toward her marrying Brain.
First of all in "The Perfect Brother," D.W. tells Arthur that she wishes Brain could live there all the time; that was while Brain was reading "Rapunzel."
Then, in "Kiss and Tell, the man whom she said wasn't Romeo stated that she wasn't Rapunzel, and then D.W. shouted, "Where is my Prince Charming?"
In "Happy Anniversary," D.W. asks Brain to examine the Tibbles' tin soldier to see if it's really made out of tin because she wanted her parents' 10th anniversary present to be official.
And finally, at the end of "D.W. unties the Knot," D.W. said that Emily could be her Maid of Honor in twenty years.