See above about nanotechnology.
Don't discount it so quickly. The potential for this field is enormous and we truly are on the edge of large advances. If you are looking for a cutting-edge field, with an almost infinite possibility of product development, this is the way to go.
Bionics - there's another thing I think is going to be big in the future. It goes along with brain implants in the whole posthumanism package. However, it involves robotics, so maybe it's not for me.
There's a lot of computer-brain interface work that needs to be done. Robotics is the gross physical end; there is endless amounts of biochemistry research and engineering design that needs to be done as well.
Quantum Computing - isn't that quite a narrow field? I don't know, but I'm guessing it will only take one group to make it applicable and affordable, and then they've cornered the whole market. Not enough opportunities within it.
Shrug. In every field, there are the discoverers, but also the people who run the businesses; the distributors, the innovators, the re-packagers. There's room to get involved in these fields in so many different ways, you shouldn't let the narrowness concern you. A lot of fields and professions are really narrow, until they become commercially viable, at which point they tend to explode. It's a good thing to be there when they do.
I don't know much about the last four (or the two above them). Can you enlighten me?
Neurobiology - http://en.wikipedia.org/wiki/Neurobiology
Quite literally the study of how the brain forms circuits to figure things out. This cutting-edge stuff that produces new understandings of human thought and cognition. Understanding how the brain works (on a complex level) is a critical step to the application and development of:
Neurotechnology - http://en.wikipedia.org/wiki/Neurotechnology
Using technology to edit and change the way the brain works. Computer-brain interfaces. Repairing damaged brains to bring functionality back. Using synthetic devices to increase processing power and memory. This is a hell of a big field and the demand for these products will be enormous once the potential is realized.
Genetic Therapy - http://en.wikipedia.org/wiki/Gene_therapy
Viral engineering - see above
Using tailored viruses and other means of inserting new strands of DNA into the cells of living patients, in an attempt to correct genetic errors. Still in it's infancy, but the potential for scientific development is immense. Cure diabetes, cure sickle-cell anemia, hack in some alligator DNA so people can hold their breath for 20 minutes. The possibilities are endless.
Advanced plastics and Metal composites - http://en.wikipedia.org/wiki/Composite_material
Combining materials such as plastics, metal, concrete, and carbon fiber in new and innovate ways to create products with the best features of each individual substance and less downsides. Lots of potential for actual physical development of products there.
Vacuum mechanics - http://en.wikipedia.org/wiki/Vacuum_engineering
We face very specific challenges when it comes to building and manufacturing in space. Zero-gravity and vacuum conditions destroy a lot of our assumptions about how things should or even could be produced. It is without a doubt that space travel, exploration, exploitation, and construction are as cutting-edge as science gets for Humans and it is a field with limitless growth potential.
Any of these would be a great way for an enterprising young scholar to make his or her mark on history.