I don't eat as many sandwiches as I use too, but banana sandwiches were part of my childhood. It may sound funny, but the way my Mom made it was to put mayo on one piece of bread and peanut butter on the other piece of bread. she usually sliced the banana down long ways and made 3 slices which fit inside nicely.

I have never thought to put another ingredient in there, but am imagining that this might be very tasty.

Through the years I have met people who only used mayo or only used peanut butter, while others had never had banana sandwiches and thought it sounded quite weird.