I find that highly personal, year-in-review content tends to get good traction during this time of year. People checking their inboxes between Christmas and New Year's are NOT going to be opening up sales pitches, but they may take the time to look at something light, easy, and personal. The novelty of it helps a lot. This is the only time of year we use these kinds of messages.
One tactic that always works is trends for the new year but also reflection on the last year. This can be new features but also use cases. As more and more B2B SaaS companies are doing it, more creative ways can be explored. One strategy we used with a client and partner is to do an end-of-the-year sale in these days. We tried it last year for the first time, and it worked surprisingly well, as we didn't expect a lot of corporate leads to be working. Maybe because not many competitors do bigger promotions in this period, it really worked. well. We are repeating it this year, and so far we see a great result again. Last year the 3rd best day of the year was actually the 31st of December, so we are positive about having a strong end-of-year rally.