Hmm, nice try with nanofluid. Besides its thermal conductivity, the fluid viscosity has to be taken into the account since it affects the mobility of the fluid in the heatpipe. Generally I don't reckon there would be significant breakthrough in the thermoconductivity and also the cost involved ain't economical. However, if you're really into the extreme, try using carbon nanotube as the heatpipe. Its massive surface area would certainly give a lot of advantage in conducting heat. By the way, I'm a nanotechnology major in the University of New South Wales. I am more than happy if you would like to talk to me about this.