Kudos to post #5. There really is not any better plot than the one suggested above. But, perhaps, one could write about the next generation and have them tested in an analogous way; that is, with a conflict between the spiritual and the material. For example, one of the children of Jim and Della acquires some material possessions--maybe the beautiful combs of Della's--but in order to save his/her betrothed, he/she must pawn them in order to travel to rescue the lover, or something similar. "My mother would understand," can be the child's final words.
I think any sequel must involve the hair combs and the watch fob being kept as treasured possessions by the couple, acting as symbols of the kind of sacrificial love that the story is all about. Maybe things can improve for them and Jim will get a better job, but they will still keep these two items as important reminders of the lesson that they learnt about sacrifice.
I think Wannam's idea is the best. It really is a great sequel because it adheres to the main theme of the original story.
Another possible sequel could involve the couple each feeling guilty because the other has given up more. Since they are so in love, it seems likely that they might each perceive that the other has given up more than they have. In such a circumstance, they might each feel guilty and maybe come to resent the other. In the end, they would realize that they each gave as much as they could and they would realize (as in the original) that it is the thought that counts.
A sequel should pick up with the same characters where the first story ended. However, a sequel usually contains a different conflict along the same theme. In "The Gift of the Magi," the couple learns that love is the best gift. An effective sequel would find a new way for this theme to be portrayed. Try turning the story on its head. In the first story, Jim and Della are poor. What if Jim found a new job and began to make lots of money? Jim and Della might be excited at first and think they would be happier than ever. But, of course, we know that money can't buy happiness. Jim and Della might eventually come across some sort of conflict in their new life and realize that they were happy before they were wealthy. Perhaps they realize that all they truly needed was the love they had in the first story. The resolution of the conflict would then mirror the first story even though the conflict is new.
Perhaps, Jim and Della could sell the combs and buy the watch back. In the meantime, while Della waits for her hair to grow, the couple could make a commitment to save every week. By the time Della's hair grows back, they could have saved enough to buy the combs again. While it is possible to purchase the combs again, the watch has sentimental value. They should try and purchase it as soon as possible. I think this would make the couple happy.
You have two gifts that, though paid for with a great price, are now virtually useless to the young couple. Della's hair will grow back so the combs may soon have a use; however, without substantial cash the watch chain will be of no use. So, perhaps you start by having both gifts being sold to purchase something together, something they can both use for now--and there's always next Christmas!