Is there really going to be a dragon on top of Gringotts at all times?
That just seems…out of place.
Not just a dragon... a very large and impressive one, it sounds like.
The first major moment when you walk into WWoHP 1.0 is Hogwarts Castle. GGGhost already kind of implied this in their post, but I look at the dragon as one of the first major moments of Diagon Alley. Without it, you're looking at just a bank. With it, you're "wowed" right off the bat.
While there's no arguing it doesn't exactly mesh with the story, to "wow" the guest as many times as possible does seem to be a very prominent goal in the whole expansion.