I think felted wool rugs would be beautiful, but really expensive to make unless you have a super-cheap source of roving or your own animals. I'd guess it takes at least 4-8 layers of wool or more to make a decent felt rug..that's a lot! (There's actually a guy on ebay who sells the most gorgeous felted wool rugs, and I've been aching to buy one from him to use as a wall decoration--search under Wool Felt Rug or Turkomen and I think you'll find him, for ideas if nothing else...he has a nice description of the basics of who makes them and how they're made, and the designs are worth looking at!).
I'm pretty much of a novice at felting, though I love it so far--I'm thinking that you might be able to something like buy old wool sweaters and cut them into little pieces of yarn, then layer the cut up yarn between a sandwich of new combed white roving and felt it together. The old sweaters would be cheaper than solid roving, and could make an interesting effect like muted colors between waxed paper kind of look??? You'd need a huge work area to do a big rug, though. Maybe you could make square 1 foot tiles of felt and then stitch them together for a rustic effect (hey, this sounds like something I have to try myself, now that I think of it!--geez I gotta stop coming up with new ideas and just start making stuff--I'm hopeless.
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Also, have you thought about the faux chenille method for making rugs? I saw this on TV a long time ago and always thought it was neat. You put several layers of fabrics the same size on top of each other, sew straight lines across all the layers about 1/2 inch apart, maybe diagonally across the rug. Then you cut through the fabric between the strips, leaving the bottom layer uncut to hold the rug together. You put it in the washer and dryer (the loose threads could clog up your washer, be careful!) several times and it rags up into a chenille look. I'm sure if you google for it, you'll find better instructions--I did a quick search and found a sample of what someone did with this method to make a coat:
http://www.kathkwilts.com/lessons/chenille.html
I love 'threads' like this one! Thanks!