The next asbestos, perhaps.
My thoughts exactly. Asbestos was once also thought to be a "wonder material". It was abundant, cheap, and had fantastic properties. It was a great thermal and electrical insulator, It was a strong fiber for holding together high temperature composites, such as brake linings, furnace and pipe coatings, etc.
It finally became known that it's biggest problem was that it was "friable". It sometimes broke into small particles with large aspect ratios - thin sharp micro-needles. These particles were too small to be stopped by cilia in human air passages, and too large to be neutralized by human microphage defense systems. They could be inhaled, and then sit in lungs creating damage for decades.
What are graphene nanoparticles going to do?