We don't know if you're trying to make your smoothie thicker or thinner and a recipe would help so we know what ingredients you are using. But certainly the obvious would be less or more of the thickest ingredient and less or more of the thinnest ingredient. Add more fruit to make it thicker? Of course the flavor will always be altered but it shouldn't be to the point of not being good by making any of these changes.
Assuming you want to thicken it. Ice can make a smoothie thicker. Small chips or cubes "whirred" into the mix with your standard or emersion blender. Frozen fruit works the same way and does not dilute the flavor.
I was hoping for some magic ingredient to adjust smoothie thickness like I adjust pH of water. I'm working on a number of different recipes, some of which I want thicker and some of which I want thinner. I suppose they just need more work.
Using the right amount of banana seems to be the key to getting the consistency I'm after, but I don't want to use banana in all smoothies. Are there other good smoothie fruits with similar properties?