Hee hee - great ideas! I vote for Kitty from the old X-men; when they demoted her to the New Mutants, she kept calling them the X-Babies - that'd be a great lead in to a story where they caught her and diapered her to teach her a lesson!
A little red Kryptonite would do the trick for Super Girl, either making her incontinent and forced to wear super-diapers, or turned into a diaper lover.
How about Ben 10 getting a new nanny alien form for Gwen?
)
And Raven and Starfire - a lost bet with the boys? They discover their mutual hidden love of diapers (both have some significant 'care' issues)? Captured by the DollMaster?