Why did the Amazons suddenly decide to break their policy of isolation to send one of their warriors to America? Because the Nazis threatened the whole globe, even the Amazons' secret island. Why on Earth is Diana dressed in American flag panties and a giant eagle? Because of some Amazon mumbo jumbo — but mostly to show solidarity with her allies in the fight against the Nazis.
Thus it gets a little tricky to transport WW to modern times while still managing to explain the combo of "Amazon-in-a-wearable-American-flag" thing. Anders has some suggestions:
1) Give her an iceberg of her own. Keep her origin in World War II and show her fighting the Nazis, only to get swept forward into this new, bewildering era at the end of the movie or TV pilot. Maybe in the 21st century, the Amazons are mysteriously gone, and she has to figure out why.
2) Give her a new reason to go to America and dress like that. Preferably some huge, terrible threat that only an Amazon warrior can overcome. Not some vague touchy-feely thing like, "people are being mean to each other." But some monstrous foe. And maybe we need an outsider to come and remind us of what America can be.
I added my own in the comments: 3. The Amazons don't send a champion out into the world--maybe globalization, environmental issues, and the march of "progress" brings the world to them. Diana leaves the crumbling colony in search of aid. You could either say that once upon a time she fought Nazis, and she's come to America in the hopes of using her old war ties to help the Amazons; or maybe this is actually their First Contact experience, and they dressed her like that after carefully studying what little they could find about American pop culture and coming to the conclusion that a short miniskirt, plunging neckline, and American flag apparel would help her fit in just fine. (Possibly they caught Ke$sha's appearance on SNL.)
Any way they go, one hopes that WW won't just pop up and remind us of "what America can be"--maybe, one hopes, she'll be amazed at the great strides that have been made since the 1950's (i.e. for blacks, gays, and yes, women).