Note: I've used both plain regular and Greek yogurt as well as substituting unsweetened applesauce for the yogurt to good results.  You can also increase the brown sugar to ½ cup, but I find that the bananas and berries give these muffins enough sweetness and it's not needed.  If you like a sweeter muffin then increase the sugar a bit.  I've used frozen and fresh berries and both work well.  I've also made them blending the wet ingredients before adding in the dry for a smoother batter, if you have kids like mine that don't like any chunks of banana visible in their muffins!