Exactly what she is already wearing. It fits her character perfectly. The Rioters that are complaining about how her outfit doesn't fit her theme simply lack creativity and have an irrational aversion to female skin, fitting the growing trend of a failing design team.
She's a fantasy police officer in a steampunk setting. Her outfit is perfect, one of the most iconic in the game and one of the most fitting to her setting.
The fact that people don't think it makes sense show more about them than her. They are too small minded to imagine that a woman in a position to look like a bombshell in a steampunk setting would do so. These people can get over themselves and find something that appeals to them instead of trying to change things that appeal to others.
Extrapolating from this, Caitlyn's outfit is the perfect litmus test for who should and who should not be hired for Riot's design team:
Anyone who lacks the creative flexibility to accept why Caitlyn's outfit is wonderful and appropriate is unfit for the position.
They might have better luck in accounting or dental assistantry, where creativity is not required.
[slayer-jinx-wink]