What should I tell current middle schoolers at a science fair to think about in terms of the scientific future? What might they expect to work on by the time they get to college?

If you want an idea of what today's school kids might work on in university, look at a current university textbook. Things don't change as fast in the undergraduate curriculum as you might think. The sequence and type of material taught in, say, introductory biology, across the United States now is very similar to what was taught 20 years ago (organic molecules, cellular respiration, photosynthesis, DNA, etc.).

On the pure science side, nothing much will change, the text books have been the same, like in Zen's answer. On the applied sciences/industrial front, though, there is much faster evolution. I can see clean energy and space research gaining strength. We still need to figure out how to dodge that asteroid coming at us at 50K miles per hour. Food for all is going to be a challenge. And epidemic containment, emergency response etc. have a lot of scope for improvement. How about making flights safer? When can we just walk out of a plane crash instead of searching for the plane for months? There are so many macro-level problems. Autonomous vehicles and 3D printing are so tiny and more industry-driven than consumer driven and will slow down.