Back in Volume two, when Yuko had folded a butterfly to track someone, she mentioned that if it had to travel farther, it would need to become a bird. The symbols that represent Yuko and Watanuki are a butterfly and a bird respectively. Watanuki isn't just a replacement of Yuko, he's her successor through and through, and one day he'll be more powerful than she ever was.
Yuko appears to find Tanpopo's form appropriate once she sees it for the first time. The little bird was born thanks to Watanuki's wish to make Himawari happy. He literally gave something of himself (i.e. his symbol) to her.
When the series gained the tagline "Rou" (meaning "cage") it was obvious that it referred to how the shop had become a cage that Watanuki was trapped in... but then he really became "the bird in a cage" when he decided the bird would be his symbol.
The Faceless Masses. The fact that people outside of the main cast and Yuuko's customers are always depicted this way foreshadows that Watanuki can't remember the faces of anyone else. This is how he literally sees them.
It may seem weird at first that Watanuki was born as a replacement of Tsubasa!Syaoran, (the whole "he'll be your parent's son") given how different they are in appearance. Even if, as Fei Wong says, Watanuki looks like a young Clow, normally it would be quite strange for a child to be born with the features of such a distant relative. Yet, Watanuki as Syaoran and Sakura's child makes sense because he does have a close relative with pale skin and black hair: CCS!Syaoran's mother, Yuelan Li. Also, one of his sisters may have blue eyes as well, which would close the circle as proof that Watanuki's characteristics are indeed in the Li's (even if recessive) genetic pool. We could say that Watanuki is the child that manifests all the recessive traits on his father's side, unlike Tsubasa who had inherited all the dominant traits (brown hair and fair, but not pale, skin: Syaoran and all four of his sisters share these); thus, they are truly "two sides of a coin".