I tried this last year with moderate success. I think I've perfected it this year. I'll just tell you what I did.
I have minimal leftovers this year. Except for some dessert, all I have is leftover turkey, gravy, and stuffing (cornbread-style). I got out a bowl, broke open an egg into it and put two ice cream scoops full of the stuffing into the bowl. I took a fork and smashed it all together. Then, I scooped two separate scoops into a frying pan (medium heat) making thin, fried stuffing patties. A little turkey and a little gravy after being in the microwave for about 45 seconds went perfectly in-between these two stuffing patties.
I was going to take a picture of it, but I ate it before I could get to the camera. It was excellent.
I certainly think other leftover items like mashed taters or corn could be added to the sandwich. A slice of cheese would also make the sammy very tasty. If you're into cranberries, I can see how that would add something special as well.
In the spirit of post-holiday hossin', what do you like to do with your leftovers?