OP, are you the veggie one, looking for inspiration, or do you have visiting vegetarians/vegans you want to cater for? If it is the latter, I would check preferences with them, particularly if you are thinking of going for any meat-substitute style things.
I have been vegetarian for more than forty years and I would feel sick at the thought of eating the loin thing someone posted above. I much prefer vegetarian food that is not pretending to be meat (so that would rule out the Quorn roast too), and I know that is common with long-term veggies rather than recent converts.
My Christmas meals over the years have featured all sorts of things, but favourites have been mushroom, spinach and nut wellingtons (but pretty much anything can work if wrapped in pasty, eg a lentil mixture), parsnip roulade, nut roast or, most recently, an Ottolenghi celebration rice cake with mushrooms and chestnuts (you can find the recipe on the Guardian website, I think).