"Underwear." Wonder Woman's look is iconic, and changing it will only annoy the masses of casual moviegoers who already have an image in their heads of the way the character is supposed to look. Letting her show her beautiful legs is NOT inherently sexist, but covering them up WILL lose sexist and/or horny male viewers (who pay money just like everyone else). The Xena skirt would be detrimental, and would probably cause a lot of jokes or accusations of "rip off." WW is a comic icon, and should not be modeled after a syndicated TV show that was popular a decade ago.
Honestly, NO ONE I know of in real life has a problem with the way WW looks. The only people who complain are a small minority of fanboys on forums like this, who think that their personal beefs and/or wishes for costume changes are what a lot of other people would prefer. Ironic, since no fanboy would call for major changes to Superman or Batman's costumes.
Some of these guys claim that audiences wouldn't take the character seriously if she wears her famous costume, even though you NEVER hear any normal people making fun of it (not even in a light-hearted Superman "underwear on the outside" way while still being accepting of it). If anything, the iconic costume is visually APPEALING. It's beautiful, colorful, and can be sexy without being overly ****ty. Since when did something no more revealing than a ONE PIECE bathing suit become too sexist and revealing again? What decade are we living in?