Wild rice or Indian rice, which is a grass seed, could be used. There is also steamed Cauliflower, roughly chopped or mashed. I have never used it, but remember reading that Quinoa can be used as a pasta substitute- this might also be applied as a rice substitute. Lastly there is TVP (textured vegetable protein), dehydrated soy protein flakes. TVP is pretty bland, but has an interesting texture when partially re-hydrated.
Another idea that I have, but have never tried is to use a mixture of sunflower seeds, crushed walnuts and almonds with some olive oil to help hold it all together.