Leah,
I know there are nut roasts out there and someone's already mentioned Quorn. I've never really had a need for a substitute per se. Whenever I would go for Thanksgiving or Christmas dinner there was plenty of food, i.e., stuffing, mashed potatoes, green bean casserole, corn, rolls, etc. etc. I just would load up on side dishes. It's what most end up filling up on more so than turkey anyway I find. I always tell people not to make much of a fuss over my diet because I will always find food to eat. Don't know if that's much help to you or not, my dear. Best of luck!