Most wizarding robes in the Harry Potter movies are a combination of bell-sleeved faux medieval robes, and old-fashioned suits. Gilderoy Lockhart looks like a 19th century dandy, Cornelius Fudge wears a three-piece pinstripe suit and bowler hat, and Remus Lupin dresses like an impoverished mid-20th-century academic. There's a variety of quite disparate looks in the wizarding world, but they all have a few things in common: mixed patterns, heavy fabrics, and multiple layers of tailoring. So even though most of the costumes incorporate elements of Muggle styles, they still don't look like something you'd often see on your morning commute. However, as I previously pointed out, they regularly rely on a late-19th/early-20th century aesthetic, meaning that the costume designer for Fantastic Beasts would be wise to go in a different direction. Personally, my first decision would be to radically alter the silhouette and fabric used for wizarding fashions overall.
The first thing you need to know about 1920s fashion is that everything uses a very flowing silhouette. The masculine and feminine ideals are very different from what we see today, right down to things like placement of muscle tone and fat, and general proportions. This is slightly more the case for women than for men, but men's suits are still pretty different in shape and cut from the way they look today. Also, the modern concept of flappers is pretty much a total fiction, which is one of the reasons why I never reviewed the latest Great Gatsby movie, and why I'm eternally frustrated by the concept of "flapper parties" and faux-1920s fashion spreads.
|from The Great Gatsby, 2013.|
|via New Republic.|
The main thing that will govern the costumes of wizarding New York is how close US wizarding culture is to the muggle world. I think it's safe to say that wizards and muggles will be separate, but how much will wizarding New York be influenced by the rest of the city? Are robes a British/European tradition that never even made it across the Atlantic? Does everyone just dress like a muggle? And how will New York's immigrant population affect its magical culture in general? I find it hard to believe that countries from all over the world will all have the same attitude to muggles, or even to magical culture in general. Surely not everyone uses the Latinate spells of Hogwarts, or dresses like the witches and wizards of British/Western European folklore. Whatever this movie turns out to be about, we're sure to learn huge amounts about non-British wizarding culture that we never even considered before.
See also: Leyendecker and the Arrow Collar Man.