Interesting story. I already had one in mind, but as it changes a lot the game mechanics, I just skipped it and reserved for one of my maps (I know most of people do not like when Heroes is converted).
In my ideas, the Christmas gifts were stolen by an child with the hearth of an Evil, who never had a gift in his childhood because he was a bad, a very bad child:

He went away with Santa all gifts.

Santa became sad, because he lost all gifts for faithful and sweet children around:

But a few courageous kids decide to help Santa and bring the gifts back:

Santa accepts the help, and, followed by the brave kids, he travels through dangers, while searching for the Evil Lord domain.

But the kids are...kids, they can't fight. So Santa is a magician, as he always was, right? In battle, gifts lay on the ground, and opening them grants with random bonuses the kids. The kids must be on the square adjacent to a gift, to be able to open it. The gift will be random, sometimes it summon a creature to help, as a squirrel, a deer or a gazelle, a few times it can even summon a bear, other times it simply grants an ability for one battle turn or a primary stat for that battle.

Happy end, of course, the evil is defeated and the gifts are back, Santa is happy and map is done

And so on (of course all battles look better on snow battlefields). Only ERM limits can stop what can be done. Sorry for posting here, but I am unable to navigate in that forum.