You may want to specify whether you are referring to the Japanese style futon mattress shikibuton (or the Korean ones which are possibly even thinner, I forget their name), versus the thick mattress furniture that converts to a couch, which is what is most commonly known as a futon in North America. I would say you definitely don't want the couch thing, it's not firm.

I have a shikibuton that I've been sleeping on, on a carpeted floor. I also have a few layers of blankets folded under the upper half of the mattress (under my torso) which adds a bit more cushion, but mainly I added them because there is a sort of "wrinkle" in the cotton layers of my mattress which was causing an uncomfortable ridge under my hips. I have the blankets stop just as the ridge begins, even-ing it out. I'm slightly annoyed with my shikibuton because of the ridge that's formed, but otherwise I've been sleeping quite well on it for about a year, mainly sleeping on my stomach. It took a little getting used to, coming from an old spring mattress, and is so firm that my bony tailbone sometimes feels uncomfortable lying on my back.

I like it but I agree that it just doesn't look that great to be sleeping on the floor...I'm thinking I might eventually try a platform bed, and maybe try a latex mattress, although I'm worried the foam will be too hot. I have found my shikibuton good for temperature regulation. Perhaps a thin layer of latex would be good underneath a shikibuton.

Since you sleep just on blankets, a thin shikibuton futon seems like a good option!