If they hadn't just done a multi-Doctor crossover, I'd like to see a regeneration story that does something timey-wimey to allow the old and new Doctors to overlap for the majority of the story, to interact directly and allow the old one to formally pass the baton to the new one, assured that his future is in good hands. Sort of like the Watcher in "Logopolis" but without the mystery. Or maybe not timey-wimey, but some kind of gradual regeneration where the old Doctor has the new one in his head and can see him and talk to him even though nobody else can (a bit like River in "Name of...").
The covers for the 1980s French editions of the Target books have the Doctor changing age, hair and costume (except he always has a scarf) but keeping the same face. It sort-of works...
That sounds entirely bizarre. Do you have a link to pictures?