Using whole grain flours will add tons of fiber, especially if you use non wheat flours or add something like quinoa flakes. Berries make good sweeteners as well as good additions to things like muffins. One thing you will run into with adding fiber is that the batter will be dryer unless you add more wet ingredients, particularly butter or oil, to compensate.
If you use coconut flour for fiber, be aware that using too much can make the result hard to sallow. Coconut is SO absorbent that it makes your mouth go dry when you take a bite and you may have to wash it down with water. Use only a little bit of coconut flour for best results, even if you find a recipe that says to use a lot.