A winter coat is one of the most difficult pieces of the wardrobe puzzle to get right. On one hand, a coat absolutely needs to keep you warm (how warm depends on where you live), to protect you from the elements, and be made from a fabric that will take a beating and still look great.

On the other, it's the first thing people are going to see you wearing for most of the season, and you may even wear it every day for months, so you want it to look great. And it has to go with everything from jeans to a skirt with heels. This is a tall (and tough) order, and I've found that having one long and one short coat for winter in the Northeast (plus a snowboarding/waterproof coat) is necessary; I don't see that I can do all this with just one. 
Rotating coats makes them last a bit longer, and gives me a chance to do any mid-season repairs or cleaning when necessary. For me, as for most women, classic colors and shapes mean that a coat will stand the test of time, but I'm always keeping my eye out for something that's a bit interesting, too. Coats can be a place in your wardrobe where you can have a bit of fun (with either color, or cut, or pattern — but probably not all three). The four below are standouts for not only their sustainability, but their easily changeable style. 
(Image courtesy of Study NY)
Study NY just opened an online store, and none too soon; this NYC-made label is incredibly hot right now. Designer Tara St. James loves to play with classic shapes and colors with an extreme twist. In the case of the wrap trench jacket (which will keep you warm with a 50 percent hemp/50 percent silk mix), it can be worn as more of a traditional trench coat, or looped loosely, as is shown above. St. James often gives the wearer something to play with in her pieces, and this coat is rife with variability. 
(Image courtesy of Kaight Boutique)
The Short Swerve coat by NaturevsFuture is the definition of "classic with a twist," highlighting designer Nina Valenti's asymmetrical style (and her always-interesting button choices, which take this jacket from cool to hot). The trim waist is balanced out by a variable collar that can be worn up or down, and helps create a flattering hourglass shape. Made from surplus wool and a surplus polyester charmeuse lining, these are fabrics that would have ended up in the landfill, but went to create this gorgeous coat instead. 
(Image courtesy of Patagonia)
This sweet fuzzy jacket (they call it the Pelage) is not what one expects from Patagonia, but while the exterior is made from a cute, curly, high-pile fleece, the interior is lined with a recycled ripstop polyester that's treated with DWR (durable water repellent) coating, which means that this furry poof of a coat will keep you warm and dry. Better than a poodle!
(Image courtesy of Beklina boutique)
This Freja blanket coat by Stewart + Brown takes the form to the next level with an asymmetrical hem, and a shawl collar (when open, this neckline flatters every face shape, when closed, it keeps you extra warm). Fair trade made from boiled wool with a silk lining by a company that takes it's materials very seriously and has been a long-time leader in sustainable design. A classic piece that will look as fresh 10 years from now as it does today.