Maybe on everything?
A neat little peter pan collar always catches my eye. I bought a pretty little stand-alone vintage pearl peter pan collar the other day, and like the idea of being able to stick a collar on anything that I already own (or even wear it with a low neckline as a necklace). Those ladies of the olden days had it right - just buy separate detachable collars and cuffs to change up a plain old dress instead of buying a dress every time you need something new.
A little collar inspiration today:
Vintage Pearl Collars
Vintage Fur Collars
Oversized Fabric Collars/Capelets