Aside from ReMind? Likely just additional cosmetics (similar to what we had in 0.2). Maybe another Keyblade. Other than that, I don't realistically see us getting much in terms of a Christmas dlc, if we get one at all.
An additional scenario in Arendelle based on Olaf's Frozen Adventure... except it all happens on the mountain because Disney said so. Also "Let It Go" plays again for some reason and it is an unskippable scene.