IMHO the key in that recipe is chopping the peppers very small, almost like confetti. It s there for color not taste. The last time I made it, I blanched the peppers and added them at the last minute, it added a bit of firmness to the softer corn.
I like the sweet potato (yam) idea, but I would drop either the stuffing or the mashed to accommodate the dish. You do need something a bit sweeter on the plate, so when deciding on a vegetable, I would think a little sweet. You could also drop either the stuffing or mashed and add risotto with onion, and peppers. Made with chicken stock, and a good cheese, it works with the turkey and the beef.
Will this be family style or plated? Are you planning on a salad course, or putting everything out at once? When I was doing the family Christmas dinner, I preferred a buffet style, and planned it so I could take off the side dishes and replace them with deserts but leave the turkey and standing rib roast for snacking later. Amazing how I managed to reduce the leftovers. I also invited family members to take home doggy bags, it helped with clean up.