In my opinion, Princess Tutu's Zwei dress with the two skirts bears some resemblance to Pierina Legnani in 1895 Swan lake, particularly to the feathered sleeves and the layered tutu, and Anna Pavalova’s dress the dying swan in 1905, with the jewel on the dress, a realist swan look, and the crown headpiece.
I also believe her white point shoes and tights are a reference to Tutu’s original design in the book as she wears white tights with no shoes (or looks like she has only white legs).
She looks like an angel or a swan in my interpretation. Fakir did see her original book design in the first series and he is into ballet like Ahiru, so he would have based her new look on those references. She looks more like Odette in this version than she did with her original anime look (That's just my opinion)
In the story, the official art, (and a brief appearance in the finale episode) Princess tutu’s original look had wings instead of arms. I believe the large wings on her new look are meant to be inspired by that, as in the opening song, the wings look like swan wings.
Her broch has made the jewel round, and the outer layer is two grey swans and a crown at the top. The two swans look like they are kissing and are in the shape of a heart. This design looks like how real swans mate and their necks form a heart. I think this is the way Fakir shows his love for Ahiru.
Tutu's Zwei design is heavily based on her first prototype design as seen in the official art by Itoh:
I really preferred the all white-theme and it reminded more of a swan or an alternate Odette, which was Tutu's strongest motif (white swan and black swan (though Kraehe was actually a crow/raven)). The major changes were the bust area and brooch, collar, sleeve puffs, bracelets and headpiece+crown. I didn't want it to look too different, so the silhoutte looks much the same.
It was a conscious choice to leave out the ribbons on her back. I wanted the wings look more prominent and the ribbons are hard to animate (saving a lot of time here).
The little crown that resembles a yellow pond lily is now looking more like white lily instead.
Tutu has a feather collar to compensate the absent necklace :)
All the aesthetics are guite classical symbols of swans, angels, love and purity. I will leave it to you all to make your own interpretations of what the different things symbolize ;)