In the smoothies I make I use bananas as a sweetener. . .
and bananas make the smoothie creamier, as I don't put yogurt in my smoothies.
If you use a ripe, but green-tipped banana the flavor is hardly noticeable, especially if you use other fruits that are more strongly flavored, like orange, nectarine/peach, or strawberries. You don't really need banana in a smoothie, but it does make for good texture. For sure avoid the very ripe, spotted skin bananas--the riper they get, the stronger they taste.
If you have a source, frozen pineapple or good canned pineapple pineapple juice are good sweeteners.