I'm sure some of these have already been suggested but wading through all the posts is a bit difficult so here's one post for several ideas. Feel free to add your own!
1) The ability to wear hats, headbands and caps with ponytails (because they are compatible). Actually headbands are probably compatible with most hairdos.
2) The ability to wear T-shirts and giant robes; they're compatible too.
3) The ability to wear cat ears and horns or any other pieces of headgear together as long as it works (panda hat and bobbles is another example).
4) The ability to wear socks *and* pants.
5) The ability to wear sweaters or hoodies and coats.
6) More fancy socks.
7) Clothes for 'incomplete' outfits: the bottom half of the keikogi (judo pants, hakama?) and dance tops to go with tutus and ballerina shoes.
8) Extend the similar section of the info page to other parts of the outfit/related outfits e.g. all dinosaur parts -- head, top and pants-- in all colors if you're viewing any of them. Tags don't always help: either because they're too vague or too precise or because they're missing from some pieces of clothing.