Our Christmas plans have already been massively changed. We normally have a large family gathering - last year was 27. The international travellers decided a while back they're staying home for Christmas so that any last minute changes to quarantines/flights/etc doesn't lead to last minute plan changes or disappointment for the kids.
We're already a household of 9 at the moment and DN will be home from uni for Christmas (he's stayed away through lockdown as he works in a care home). SIL and her kids are bubbled with us, but she'll likely be spending Christmas with her Mum.
Whatever we do will entirely depend on rules, but also on how careful people have been even if it's allowed to have a bigger gathering because DD4 is extremely vulnerable.