Yay smoothies! I drink a smoothie every morning. For what you're starting with, I would replace the water with whole milk, which will add substance and protein (I don't use protein powders). If you want more, oats are a good addition as zygote suggested. You'll want to play around with the ratio, as a lot of oats can make it pretty thick. If you want more fats, peanut butter is a good addition, or just olive oil. I also recently started adding a couple raw eggs to mine (which replaced the olive oil), for more protein, and can't tell the difference.
For a reference, here's my current smoothie recipe, but note it is primarily for calories (works out to about 1000 cal and, IIRC, 37g protein):
1cup milk
1/2 cup rolled or quick oats (not steel cut, they don't blend as well)
1 banana
2 heaping cups frozen berries, and/or mango, and/or pineapple, but trending towards blueberries and strawberries recently, as they've been fairly cheap
2 Tbsp peanut butter
2 eggs
Handful of spinach, just because I never eat enough greens.
Fills a quart jar just about to the rim.