My Osprey Farpoint 55 worked very well on this trip — it was large enough for my clothes and tripod and had a detachable daypack for carrying during the day (since I often needed to carry extra layers and gear).
No cotton: Cotton absorbs moisture and quickly becomes a fabric that makes you colder rather than warmer. Avoid it in all of your layers if you possibly can.
4 layers of thermals: Thermal underwear is one of the most important things you’ll take with you. I had at least two layers on top and bottom at all times — a layer of 200GSM merino wool and a layer of polypropylene.
2 bras: I recommend supportive over cute, since it’ll be hidden under a pile of other clothing and you’ll be doing a lot of activities that cause a lot of bounce, like snowmobiling and dogsledding.
Gore-tex gloves/mittens with glove liners: Gore-tex is one of the warmest, most waterproof fabrics you can find for winter gear. It comes with a big price premium, but even in -33°, my Gore-tex mittens kept my hands quite warm.
Fleece neck gaiter: Not only does it let you keep your cheeks, nose, and mouth warm while doing things like dogsledding or snowmobiling, but it also helps you breathe during the cold nights.