I typically bake cornbread rather than muffins because rectangular pieces store more compactly in the refrigerator than muffins do. Usually I add a little maple syrup. I have heard of blueberry corn muffins. Today, on a whim, I subbed apple cider for half the milk, cutting back on the total liquid volume by a third, and added the same volume of mashed baked kabocha (butterCUP) squash. Also a glug of maple syrup. Before baking, I topped the batter with a diced peeled Empire apple. The apple didn't sink, to my pleased surprise. I like the result a great deal. Kabocha is drier than butternut or acorn, and tastes very much like sweet potato, so I'm sure mashed sweet potato would sork as well.
The combination of apple and corn isn't common but should be. Both are such fall harvest icons.
I think just about any dried fruit would be good in a sweet cornbread/muffin, too, and some walnuts or pecans would play well. Any other ideas for sweet additions to corn batter?