I know you've decided now, but depending how much effort you want to go to, my former Christmas day stalwart was this:
https://theordinarycook.co.uk/2011/10/0 ... ellington/With added porcini.
I think you want something with a bit of protein to balance the meal out?
It is a tricky one, and I think there is a tendancy to veer towards meat subs perhaps because of the protein factor. I do like a good nut roast, but a good nut roast is quite hard to come by, and not as easy to make as one might think. (Dry and crumbly ones are...)
I just saw a recipe for a mushroom and stout mini suet puddings (Mushroom are another theme. I guess they offer a bit of umame.) Which remind me, there is Mildred's recipe for individual savoury puddings that I've been meaning to try for ages.
“In the midst of winter, I found there was, within me, an invincible summer." Albert Camus