Okay but the same can be said about superman why doesn't he have more than one clothing option? Why did his parents only give him one? I'm not saying that your idea isn't a good one just seems arbitrary that she would constantly be quick changing between battles.
In MOS they make us accept the S suit by saying it's the general clothing style of Krypton, and that Superman wears it as a remembrance of his place of origin.
Plus, lets look at WW's costume changes over the years. Why not honor that with multiple looks in her films? Not to mention the added revenue that gets brought in from multiple action figures/dolls/statues. I even think that WW could bring in money from fashion if you had her in more "normal" attire like the white Emma Peel get up or the Jim Lee jacket. Replicas could be sold in stores or be available online.
Plus, if the Diana Prince ID is not going to be a big factor, as it has not been for over 2 decades, then she's stuck in only the one outfit for either the entire time she's on screen.
But lastly... WW is a kick ass, powerful woman in the mode of a Lady Gaga, or Beyoncé or Madonna. Do you think any of them would stand to wear the same outfit everyday?
PS: This also satisfies EVERYBODY. In S/B or a solo film, or JLA, fans will get to see a version they like at some point. And it adds a layer of realism. Let the boys do the traditional one outfit approach. WW wants to cover her legs some days, and others she wants to show those gams off. How can she do that? Simple: She's a woman. She's entitled to change her mind.
