I have found than when I can get them avocados are very nice about adding a creamy texture to smoothies, and of course there are many, many different fruits and things that can be used.

I have been overall getting away from making so many smoothies, partly because there is too much temptation for me to go overboard with sweets (since even naturally-occurring sugars in organic fruits affect the blood sugar) -- and partly because I'm just not feeling very attracted to smoothies for the past year, with the exception of cacao smoothies, which are pretty dense the way I make them with about 3-4 TBL home-powdered cacao, a couple handfuls of brazil nuts, a handful of pecans or other nut such as cashews or walnuts, 2 TBL honey (already too much!), stevia (I like it sweet), some vanilla bean or vanilla extract and a pinch of salt. Sometimes I add bee pollen or other superfoods. This is a very highly concentrated drink. I used to mix just the cacao and nuts first with about 3 c. water and strain it, then return it to the blender to add the other ingredients to give me a creamier and more milk-liked texture, but for fiber and economy usually now just drink it without straining it as it really isn't bad that way to me at all. Adding an avocado to this would be great.

Another thing that adds a sense of creaminess is cashews, but I generally avoid overdoing the cashews and try not to have more than a handful on any one day. Another possibility is fresh meat from young coconuts. Hope that helps!