Avocado substitute in blueberry muffin recipe
In general, when making muffin recipes, you can replace around 1/2 of the oil (sometimes up to 2/3) with applesauce or mashed banana without significant problems.  I don't know if you could get away with it in this particular case, as avocado would be a solid fat, so it might adversely affect the texture.

I'd personally try replacing the avocado with either mashed banana, coconut oil (mashed up some, but not melted), or a combination of the two.