William may well change what they do. Personally I’ve always thought its odd they leave Sandringham which is close to beaches for their winter holidays and not summer but thats just my opinion. William may well have a huge change about and I certainly think Catherine’s family will become core to their Christmas. Likewise I think he is close to his cousins so they would be welcome in some way- alternate years etc but Middletons every year. Who knows, whatever we think is pure speculation. Personally with William I don’t think anything of off the table and it of course depends at what point he becomes King.
Its worth noting some years with the late Queen were much quieter and smaller than we see now- her, Philip, her children, James and Louise and Beatrice and Eugenie. Its only as the late Queen got older, had great grandchildren etc that the numbers at Christmas really grew and grew. It’s natural from time to time to have a change about- just as the late Queen did when it moved from Windsor back to Sandringham (despite the fact she always then travelled on to Sandringham for her break).