So funny, I'm literally just writing a post about smoothies and what to use instead of banana. Frozen mango, zucchini, cauliflower or avocado would be my top choices for this one. Or a combination. They all give a really creamy texture. If you don't use the mango, you will need to add extra sweetener though to make up for the sweetness missing from the banana.
I do have some banana-free smoothie recipes. This lemon cheesecake one tastes like dessert: https://avirtualvegan.com/luscious-lemon-cheesecake-smoothie/
And there's this one although it does taste quite green: https://avirtualvegan.com/kale-apple-smoothie/
And this one: https://avirtualvegan.com/cranberry-walnut-apple-smoothie/
Hope that helps!