A lot of other quick bread recipes substitute banana for part of the oil, so just using an extra banana works in place of some. One of my banana bread recipes uses 4 bananas in place of the usual 3, and yogurt in place of most of the oil.
If the concern is the type of fat, you can use mashed avocado in place of butter. It sounds weird, but it really works with any tropical fruits. It's good in banana bread, and fantastic with citrus.
When I lighten baked good recipes, though, I don't like to take out all the oil, so I take out a bit of the sugar and leave a little fat. With banana bread, if you bake the bananas until they're very soft before you peel them and then add a little cinnamon, nutmeg, or vanilla to the recipe, you can leave out about half the sugar.